Players also find it fun and satisfying when an FPS player can carry five large weapons, with 100 pounds of ammo for it, run while carrying all that 20 mph in any direction without getting tired, pick anything from the floor without slowing down or ceasing fire, etc. A realistic shooter would be much harder, and having to limp slowly after taking a stray bullet in the leg would suck.

And people play for fun, not for feeling the misery of war. Or, in that case, of the slow and restricted early medieval life.