In fairness, at a mega-corp there is very little data on whether an employee would be successful in a different group. Some groups have bad charters/bad team dynamics/bad skills match. An internal transfer has a huge amount of visibility into these aspects when they transfer, much more than an external hire would.

Its a pretty good deal to suggest to employees that they find a new team rather than performing a layoff, I wouldn't be surprised if the profitable/successful teams can still add head count for people they like.