My unofficial answer: Because it would destroy play balance. If the USA was able to directly transfer to England, you'd never see any American troops. Likewise, all the Italian points would be transferred to Germany.
A more historical answer would be that soverign nations, even when at war, maintained separate militaries. And in the game, MPPs are used (for the most part) in purchasing military units.
Now, you could consider Lend-Lease to be a transferral of MPPs from the USA to Great Britain. (And many opponents in Congress screamed it was just that.) But then the game includes a way to disrupt that, via submarines.