Picking up and knitting collars

I like to knit sweaters top-down in the round, and I’m trying to figure out if there’s a reason that the collar instructions always involve picking up and knitting from the cast on edge. It makes sense on a crewneck that is knit flat for awhile to shape it before joining in the round, but on a boatneck or round opening where the sweater is knit in the round right from cast-on, is there any reason not just to begin with the collar (ribbing or garter or some such), then start the shaping increases once the collar is the desired width?

I wouldn’t swear to it, but I think it has to do with the way it lays. If you do a turning row it might help it to lay the way you want. I think a swatch might be a god idea. Do a bit both ways.