Like a lot of people here, I have absolutely no problem with bittersweet endings.
What I have problem with is when some of my choices and character arcs are being completely ignored. Like romanced Astarion starting to burn and having to run away from the docks and companions WHO praised him for becoming a better person suddenly crack underhanded jokes about it. Sure, have Astarion slowly scorch in the end, but have our MC react to it appropriately. When I heard Minsc's line "BOO WILL BRING YOU A SUNFLOWER!" I legit wanted to grab that hamster and violently ram it up Minsc's throat, so he'd shut up. It's how bad all the mocking is really. And the fact that the MC just stands there with a smirk on their face doesn't help.
The second reason is they see this game as the end. The story stops when the credits role. I get it, that is the way a lot of games work today. However I am "hopeful" that Larian loves the ideas od DnD as much as many of us and those of us with a deep DnD (or any well done table top RPG) background know that loose threads, unresolved situations, often lead to entire new adventures.
Do you know what all those games also have? Proper epilogue, ending slides what have you. BG3 currently seems to be lacking those.
Do I want to know what Kagha did in the Grove after she was given a second chance? YES.
Do I want to know how the Myconid fared after my adventure in the Underdark? YES.
Do I want to know what happened to Lathander's monastery after we cleansed the Creche of all Githyanki? YES.
Do I want to know what happened to the Shadow Cursed Lands once the curse was broken? YES.
Do I need to continue giving examples for just some throwaway lines of how our heroics/villainy changed the lives of random NPCs/animals/places? YES. Uh, fine I will stop.