Sigh.
Reality = Dude attacks another dude in plate armor with sword. Sword attack glances off. Repeat. Same effect. Repeat. Same effect. Repeat ... you cannot deal any damage with a standard sword against plate armor. You have to find the gaps (which not always exist by the way, some plate armors completely protected the whole body) with halfswording or use a topheavy weapon so you can deal enough blunt damage despite the plate armor to for example manage to break bones beneath the armor.
D&D = Dude attacks another dude in plate armor with a sword, rolls high enough, and hits him for X damage. There is no such thing as hitpoints in reality.
Reality = Plate armor was the ultimate armor at the end of the medieval age. One plate armor can be wildly different than the other. There was a ton of other armor types as well, but wasnt really leather armor. Leather does not even remotely compare to steel in robustness.
D&D = Uses tons of armor which are grouped into light, medium, and heavy. Within any of these groups, all armor have the same functionality. You can immediately wear a plate armor dropped by an enemy (in reality plate armor has to be tailor made, since steel plates are rigid and if they dont fit, the user can be cut or may be unable to move etc).
Reality is not like D&D at all. The point of D&D is not to reproduce reality, but to have a balanced ruleset that is a fun base for games. Asking for realism is thus not serving any purpose. This by the way is true for ALL game design. Games have to work as a game.
