Because It is a ROLE PLAYING GAME. Not an min/maximizing Pun Pun Building game.
Low int runs on Fallout 1/2 aren't optimal but are fun
Nosferatu runs on VtMB are not optimal but are fun
Trying to be a drwarf mage on arcanm is not optimal but is fun.
Solo challenges for BG1/2 aren't optimal but are fun
(...)
This excessive focus on balance is why modern games has so many inconsistencies between LORE and MECHANICS.
It being a roleplaying game isn't an excuse to have imbalanced mechanics, especially in games where several players might be involved. Like BG3, given it has multiplayer. I really don't see the problem of giving, for example, each race both something to incline them towards a specific playstyle and something that is universally useful if they choose to play against the archetype. Like the half-orc, as I explained above. And I really don't see why keeping some classes from completely dominating others is such a bad thing either. That isn't about min/maxing, but rather about not punishing players who want to play something unconventional because they think it sounds fun.
That being said, the 'game' part of roleplaying game indicates that we're talking about a collection of challenges meant to be overcome. Challenges designed with some guidelines on how powerful one can expect the player to be. Making it so that certain playstyles trivializes said challenges, while others can barely make it past, just seems like bad game design to me. The difficulty of a challenge should center around the player grasping and utilizing the options at hand.
Of course, if we're to talk about lore and mechanics interacting, I would say that keeping some playstyles from dominating the others certainly makes more sense of most fantasy worlds I'm familiar with. It really doesn't make sense that, for example, clerics in 3e aren't ruling everything, because it doesn't take many levels before no one else can compete. The more powerful something is, the more work has to be put in to explain why it isn't used to take control successfully. Off the top of my head, only the Dragon Age series has addressed this imbalance within their world in mainstream fantasy as of late.
In fantasy, you can (and should) ask what the people with knowlege and resources (rulers, wealthy merchants, priests etc.) are doing to protect themselves from...
... mind control and magical charm.
... invisible thieves and assassins.
... teleporting thieves and assassins.
... cheap materials disguised as expensive materials (lead to gold etc.).
... shapeshifters or magical disguises.
... creatures unkillable by non-magical weapons.
... semi-immortal creatures, like liches.
... giant monsters, like dragons.
... undead rising from their graves.
... ghosts floating through the walls to kill you.
... etc.
I'm sure people could add a million more things that would have to change in a great many fantasy settings for magic, even magic that isn't necessarily stronger than just stabbing someone, to make any sense with the lore as it is presented, let alone the absurd power it is sometimes meant to have. If you want to complain about inconsistencies between lore and mechanics, there are way bigger problems than "magic isn't overpowered enough" or some such to be addressed.
