RID column in the database should be auto-increment

I need to import the relationships from a CSV and it should have worked perfectly fine had the "rid" column been auto-increment. I went into the code and saw that the module uses db_next_id .. If you follow the link to db_next_id, it says that the function should be used only if for some reason the column cannot be made serial. So the question stays: Why cant we make the column auto-increment?