It is a science fiction setting with advanced futuristic medical technologies. Things like growing a new limb in a tube from a stem cell, having an artificial immune system, and such, are possible, among the relatively simpler to implement cybernetic solutions, that are in no way worse alternatives.
In such a setting, how can a character retain visible damage they've acquired - like a blind eye or scarring - without it just getting fixed by their advanced medical tech?
In my opinion, the simplest possible explanation that can be used is economic. I.e., the fact that a society as a whole is in possession of medical technology that enables what you described does not mean that all members of that society have access to such technology. Arguably, access to the most advanced elements of the technology is very expensive, so de facto only the richest members of society, or only the upper and middle classes, can benefit.
Even nowadays this is exactly the situation, i.e. the cost of the latest and most effective therapies is usually very expensive and thus access to them is reserved for the few. What's more, the cost of treatment and financing models in different countries vary dramatically, and for example, what is available to every member of society in Europe and is financed by universal insurance is completely inaccessible in third world countries, but also, for example, to poorer members of society in the United States - those who do not have adequate insurance.
So it's enough that your characters are members of the middle or lower class and simply can't afford to undergo even relatively technically simple (in your world's terms) but expensive therapies to correct their physical defects.
This signifies an inability to heal normally.
The unfortunates that react this way produce massive lumps and bands of bulging scar-tissue because of an overgrowth of granulation tissue when healing occurs:
From: dermnetnz.org Reproduced with permission from ©DermNet 2024, fair usage as educational.
The slightest scratch (from a vaccination in the picture above) can leave unsightly purple/brown bulges, way, way bigger than the original injury site was.
There is no cure, and this makes it all but impossible to remedy the cosmetic defects and useless when eye-surgery is needed.
Gene therapy at near-conception time, or genetic screening would prevent the condition if those who possess it were not allowed to grow to term, but a widespread medical-scandal decades ago where money was being embezzled by a screening-test supplier supplying sub-standard products all meant that many were allowed to be born with this (and other...?) avoidable genetic conditions. A warehouse manager was blamed, just before arrest they mysteriously died in a brutal accident. An underappreciated janitor unexpectedly retired to the Bahamas.
I can think of several possible explanations. None of them really involve science...
The scars may be reduced, but people get better at spotting them. If you think someone has had cosmetic surgery done on their face, you look for it. If you live in a society where everyone strives for a flawless skin, any deviation from perfection stands out.
Some scars may not be considered as disfiguring. Maybe there is some backlash against the cult of the perfect complexion. Perhaps people do not want to look like everyone else. Maybe security and face recognition may play a part in this.
It may be seen as wasteful. Suppose it is always possible to undo any amount of scarring. Those last few steps towards perfection may take a lot of time and money to achieve. If anyone can have a perfect skin by spending enough money, or spending the state's money if you have public health.
One possibility is that the scars are a result of dueling. Much like dueling in Prussian countries in the 19th & early 20th Centuries, the scars are seen as a sign of masculinity, or obedience to cultural norms. With remarks like "of course we could have removed the scars, like some average plebe, where would the honor be in that sort of act?" followed by twitchy hand motions towards the blade at their hip.
Alternatively, some people are allergic to the treatment that removes scar tissue. The treatment stimulates stem cells and in rare cases, the people involved are allergic to the drugs used to de-differentiate stem cells. Instead of allergic, your story might have the medicines work on 99-ish per cent of the public with some ethnic minorities having some genetic resistance to the medicines/drugs.
Sure, you can get rid of scars. Even today, plastic surgery has (some) ability to deal with them. However, except in cases of severe scarring (such as from an acid attack, for example), non-narcissists generally do not consider it necessary.
In the "Nick Seafort" saga the very religious main character has to break his oath to safe his space ship. In the process he gets wounded and a very big an ugly scar remains in his face. Though every doctor in that time could regrow brand new skin cells for him and get rid of that scar, he keeps it as a reminder of his broken oath.
But he is also a war hero (or at least he gets presented as such) so naturally the youth imitates him, which leads to a fashion trend, where kids and young people intentionally scar themselves and refuse mending.
Later he gives in to the pressure of his superiors and has the scar removed in order to stop that insane trend.
If you just need one character to keep a scar or disabled eye you can make up a reason for them to have that. Also gives a nice background opportunity.
If you need multiple characters you could also go with a popular person doing it for some (maybe unknown or later revealed) reason and have people scar themselves for fashion. Also gives a nice world building opportunity.
With medical abilities like this, lifespan has likely extended - dramatically. People are no longer considered adults until they are much older; 30, 40, or even 50. The state is paternalistic and doesn’t allow people to make poor medical choices such as having disfiguring scars until this age.
With no age-related declines, experience becomes an increasingly valued commodity. Also with all that health, everyone starts to look the same after about 21. How do you tell if people are grown up and experienced enough to talk to, socialize with, or trust?
So adults have gotten into the habit of leaving visible marks of their injuries. They always know if the inconvenience is too great, they can have them fixed. But they are a badge of experience. Outfits might even be made to deliberately show the scars to emphasize them.
Young people might even intentionally have scars added cosmetically when they are old enough to control healthcare choices. But to be CAUGHT doing so is extremely shaming.
There is always an exception to every miracle cure. Some injuries simply don’t respond well to treatment. They are rare, but happen due to severity, genetics, exposure to rare chemicals, and so on.
Only, once your lifespan starts to be measured in centuries, more and more of these rare, odd, very difficult to fix issues show up.
In Larry Niven's Known Universe series, the humans had a drug called boosterspice. It healed people completely, including making their scars go away.
The Kzinti of that universe had their own version, but it didn't heal scars. They did this on purpose. Their scars told their story, and served as a form of identification and individuality. Covering up one's scars would be like giving up the memory of life's experience. It was considered a sign of weakness.
When someone figured out how to replicate the scar healing effects of boosterspice for Kzinti, the guy they used it on saw it as a punishment and an offense. His own children would no longer recognize him, and he had to give up all of his social status.
So, maybe, it doesn't heal scars because they just like it that way.
Maybe there could be some conditions for the treatment that often go unmet. A patient would need to seek treatment very quickly to fix some major injuries. People could create weapons with effects that specifically counter these technologies. Maybe there's something about the character that makes them distrust the medical technology or causes them to be denied treatment.
What are the details of the technology? Today we have vaccines for polio but not the common cold. Maybe some types of scars can be healed but others not. Just because a person or a culture can do amazing things doesn't mean they can do EVERYTHING. It would be nonsense to say, "Oh, you claim to be able to predict the weather? Then why can't you predict tomorrow's winning lottery numbers?"
As others have pointed out, some people may prefer to keep their scars as symbols of their manliness or to look tough. Some may see having scars removed as pointless vanity. Like I'm going bald. There are treatments for baldness, but I don't care. Once we saw an ad on TV for some baldness treatment and my sister in law said, "I'd be worried about a man who cared that much about his hair."
Others have pointed out that the process may be expensive or otherwise limited and only available to the rich and powerful.
Maybe the treatment is long and tedious, or painful, and some choose not to do it. Like, suppose someone loses a finger in an accident. He is told that they can grow him a new finger, but it will take years of treatment and be excruciatingly painful. He might decide it's not worth it. If the same person lost an arm, he might decide that is serious enough to be worth it.
Some people may just not bother or never get around to it. If the damage is minor, it may just not be high on his priority list. I have some prescription meds I'm supposed to take, and I often miss them. Not because I can't afford the pills, or because they have bad side effects, or any other substantive reason. I'm just lazy and forgetful.
One option is to achieve most wound healing by the use of general-purpose tissue scaffolding gel (in fiction, ususally called Omnigel).
This is actually an idea that we are slowly putting into practice IRL. Spray scaffolding foam into a wound, fill it with extra nutrients, collagen, stem-cells and growth enhancers, and the body will spread its cells into it, filling the scaffold.
We already managed to "print" hearts and livers that way, and even produced some skin.
The problem is: if the cells grown through and over a scaffold of quickly applied foamy gel, the resulting tissue will look not better than the blob of scaffolding itself, a gnarly patch of flesh visibly different than the rest. Its the equivalent of having your wound filled with insulation foam that then magically changes into meat.
Flesh can be regrown fast, or it can be regrown properly, but not both at the same time.
What that means, is that if your future soldier, cyberpunk cop or a warrior gets severely injured, they can almost certainly save their life by filling the wounds with omnigel foam, at the cost of making them look like Frankenstein.
This applies double for anyone with cybernetic implants.
If you want a robot hand, well, the connection point will be basically built out of omnigel scaffolding, so that regeneration outpaces implant rejection, which means it will look like melted Plasticine.
