What you say is completely true. If you were an intern at Microsoft you can be pretty sure you're returning to that team (unless there's a reorganization). With Google you have no idea what team you'll end up on when you're evaluating an offer.

However, once you accept a Google offer you're given more choice as to what team you'll be on. With Microsoft (if you weren't a returning intern) you're given very little to no choice and it can be quite arbitrary where you end up.

Certainly both of these systems have massive flaws, but I think the Microsoft system would lead to far lower retention for non-intern college hires.