Because DnD 5e is not a perfect game system. It can actually be really boring and repetitive at lower levels, especially if you're running a martial class, and it has some obvious imbalances inherent in the design that have never really been fixed like DEX just being an all around superior attribute than STR or just.. The entire ranger class.
That isn't to say I want BG3 to radically be different from 5e of course, because it does have its merits and I think it is a fundamentally strong baseline to start from.
But it also doesn't mean I want to religiously adhere to the 5e rules perfectly with no deviation.
And that also isn't to say I agree with all of Larian's changes. I don't think cantrips should be able to cause surfaces as a by-product of attacking and I definitely don't think you should be able to just jump out of someone's attack of opportunity range freely like you do currently. I'd also like them to tone down the number of bombs and acid vials enemies have. Not remove, just tone down to make them more rare.
So I am firmly in the camp of "It is okay to deviate from 5e as written. We just need to work with Larian to find that sweet comfort zone".