Well the answer to why not is that if you're also able to craft magical weapons, then as soon as you're able to do that, the normal weapons and any ingredients reqauired exclusively for them become useless, because why would you craft and use a normal weapon when you can craft and use a magical weapon that's just objectively better. That means the devs are investing time and money into content that's going to see very little use, as well as cluttering up the world and system with undesireable stuff that players don't want.
Also your argument about crafting that if a player doesn't like it they can just not engage with it rings false for several reasons. The first is that, as it was pointed out, unless the game explicitly says that players don't have to engage with the crafting system, players won't know that they have a choice, and they're going to waste time gathering up all the overabundant resources Larian has dropped in the game, which slows down gameplay. And honestly, do you actually think Larian will explicitly say "don't worry about crafting, it's entirely optional for you to do"? Why would they invest time and effort into a system that can be ignored without impacting the game experience? Why would we assume that they would tell us if their crafting system was totally optional? I can give you a reason they would not do that; because they worked hard on it and want players to interact with it. If they didn't want players to interact with it, they wouldn't have included it in the first place.
Also, the idea that crafting will actually not impact the game experience and it's a thing that'll be completely optional is just silly. On lower difficulties you'd probably be able to ignore it, but for anyone who wants to play on higher difficulties, I've yet to see a game like this where part of the higher difficulty doesn't involve making serious use of items and constantly having the best possible weapons and potions and items, which pretty much always involves some level of crafting. Then there's the fact that if the system exists, then the devs are going to let that impact other development aspects. It'll impact loot at the very least, because crafting supplies are going to be all over, and it's inevitable that players will pick some up even without going out of their way to get it, which will clutter up their items. Then we have no idea how it will impact power scaling. Will there be stuff that can only be crafted? Does all this mean that someone who dislikes the crafting but is finding the game too easy at the level which allows them to ignore the crafting will have to pick between two unpleasant experiences?
