Basically - it comes off that Larian have already decided, for themselves, what the PC's character is like, or, rather, they've already decided what the character representative for each of these classes is like - and either you play the character role they've given you, or you don't take any of the options associated with your class at all.
Just speculating, but I wonder how much it has to do with the origin system. If BG3 is anything like D:OS2 then race and class reactivity might have been written with particular character in mind - have wizard lines fit Gale, have Warlock lines fit Wyll, make barbarian/halfling lines fit whoever will be their origin. After all origins will most likely use more or less the same conversation system and race/class options will be easiest way to characterise them + some unique options no doubt.
