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.