It's hard to pin any one thing as the reason why BG3 feels like a shadow of what it should or could have been. You could go on for hours about what's done right, what's done wrong, and everything in the middle. Not everyone will agree. I don't like this game, but I don't understand the criticism about how 'serious' it is. BG3 takes itself quite seriously. I just think Larian is laughably bad at writing anything serious.