Because the official companions have a unique backstory, unique questlines, they have relationsships and banter, some of them even have unique subclasses and abilites.
The price for full customisation is that your hirelings have none of that. But hey: you can give them a custom portrait and name.
Yeah I kind of wish in the first game there was set paths for all levelling up and that for each NPC so I could just let them do their own thing.
BG2 would be a worse game if you could just let Anomen be a bard, or Edwin could be a fighter.