The people arguing in favor of the writing of BG3 fail to address the fact this game's writing and story are mostly boring with some cringe mixed in.
This generic subjective criticism could apply to literally all the games you listed as examples of "good" writing. I was far more bored with Mass Effect, KOTOR 1, and The Witcher than I ever was with BG3, and these days I'd probably cringe harder playing Fo2.
I don't see anyone arguing that BG3 is some kind of revelation in writing, but it's a basic heroic fantasy adventure story that mostly sustains itself until the third act, when it falls apart, and has some superficially funny and engaging characters (which is more than BioWare have ever managed). For me it's basically on par with BG1 and BG2 in terms of writing, except with significantly more interesting party members and other NPCs. I also thought the hook of having the mindflayer deceive you in dreams was a decent idea even if, like most things, it fizzles in Act 3.
I think the reason you're getting this reaction is because of the games you listed as alternatives - if you'd said "I personally found BG3 boring and think it could have been better written" (or even "BG3's writing is mostly shit") then everyone would probably agree, but if you come in saying "BG3 cannot compare to the objective classics like KOTOR and The Witcher and Mass Effect!" then everyone's going to say "but hang on, they're shit". All those games got torn a new asshole on this site when they were first released.