The balance of Offence vs Defence swings the other way
Right now, the problem is that an armor that can stop a bullet in its tracks is expensive, heavy, inflexible and generally only good for one or two hits with low-calibre weapons.
Weapons technology has vastly outstripped personal body-armor to the point where we no longer defend against direct attack.
Instead, we defend against secondary effects. Shrapnel, debris, glancing hits. Things that would cripple but usually not instantly kill.
This, incidentally is the ideal-case for a Storm-trooper's plasteel armor-plate.
Remember the prison-block in A New Hope? Remember how fast the fire-fight turned it into a storm of debris, shrapnel, smoke and alarms?
Those storm-troopers might die in a single direct-hit, but they shrugged off the shrapnel caused by blaster-fire hitting walls and blowing up nearby hardware like it was nothing. And they started shooting before the smoke from breaching the doors had cleared.
Those helmets have all kinds of respirators, filters and optics to let them fight in hostile environments where a lesser-equipped person would be blind, choking and probably dead.
We also never see a clear over-penetration of the stormtrooper armor. Plenty of burn-marks and troopers falling over, but plenty of times a trooper was clearly still alive after being hit. For example one trooper fell off a catwalk when directly-hit and was clearly screaming as he fell.
One imagines that the storm-trooper armor is actually excellent as a defense against lighter blasters (such as their own E-11 carbines and hand-blasters) and most of the storm-troopers we see shot are actually alive, just knocked on their asses by the impact and burns they just received.
So back to the offence/defence tradeoff.
Stormtrooper armor is very light-weight. The clue is in the name "Plasteel", a lightweight polymer with properties reminiscent of steel.
If it weighs anything like say.. ABS, or a thermally resistant resin, it could be very lightweight indeed.
An armor that defends against most non-military small-arms well enough to prevent the trooper being killed in many/most cases, and can turn a military-grade glancing hit into something survivable, provides NBC protection, can see through smoke and gases, shrugs off shrapnel and acts as highly functional riot-gear?
Now imagine what that would look like in real-world modern terms.
A suit of body-armor that can stop pistol-caliber hits entirely (even if the wearer gets knocked off their feet), and can turn a killing blow from larger caliber weapons up to .50 cal into something survivable.
We don't have the technology for that right now, but if we did, we might see the return of the suit of full body armor.
So why would a future military have standard issue armor that covers the entirety of the body, especially when it literally never does any good? $\endgroup$