I'm gonna have to side with CuriousBlueAbra here.

While the mechanics of the game are certainly influenced by real life, ultimately the game takes liberties in order to create a somewhat ballanced and fun gameplay experience.

Plate armors do get a bonus against slashing weapons, but not so much as to render swords useless. You can rationalize the hitpoint system by saying a high level fighter dies from an arrow to the face as surely as a low level fighter, but ultimately that scenario doesn't come up, because it'd be boring if your high level fighter just died from a stray arrow in a chaotic battlefield that nothing short of precognition could've predicted.

Weapons canonically are often capable of more than mundane physics would allow. By the end of the game you've got a small arsenal of magical weaponry stashed away in a sack that can magically hold more than its size should allow. The in game description of 2 handed weapon style even explicitly lays out that these weapons just coincidentally happen to be more heavily enchanted than their one handed counterparts for reasons. 2 handed weapons have pretty much never been a terribly practical thing in real life but hey, it's a videogame. People wanna take a big sword into battle and Baldur's Gate ensures there are big swords that'll make it worth your while.

Off the top of my head, here's a couple ways we could take influence from The Forgotten Realms to make Crossbows a more appealing weapon. Dark Elves are known to keep two tiny crossbows hidden under their cloak so that they can surprise an opponent with a poison dart fired from each hand. Right there you've got dual wielding crossbows and you've got crossbow surprise attacks. Bada Bing Bada Boom.