I mean, a dirty peasant, a bartender and a merchant wouldn't survive a day of BG3's exposition sequence. Thats why I think it makes sense that all origin characters are rather special to begin with, some more than other. Survival of the fittest, no?
Can't answer your main question though. I can really see how introducing companions as possible main characters can waste a lot of resources for no huge benefit, because we'd know their story regardless. I guess one might say it was done for those who don't like to bother creating their characters or struggle with the process, but seems like a weaker argument to me. So, dunno
