D&D combat rules are not realistic. They just aren't, and it has been a feature of the game since the very beginning.
Weapons do not act like they should. There is the much-overlooked sling (should be superior to a shortbow), Armour Class is clumsy and armour protection versus different weapons has only been tried back in 1e with that awful chart that most people I knew ignored. HP are awkward when you start analysing how damage is taken and resisted. And so on and so forth.
The point is that D&D itself does not offer a realistic simulation of combat, and Larian have largely used the D&D5 rules to create the game. Yes, spears ought to be better, but so should bows. Swords should not be able to cut through steel plate, enemies ought to surrender or fall crippled long before death, wearing 'studded leather' should be restricted to specialist adult clubs, etc etc.
However, in D&D, they are what they are. You either abide by D&D rules with all their peculiarities and faults, or you throw out the D&D aspect and make just another FRPG.