Alot of people CHEAT for them, so if it went to out of local and to soemthing bigger, peopple would wonder about how this person got that many things/achivements ect.
I think they should stay local.
Just my Opinion
On the other hand, cheating for achievements only matters if you care that someone else has them. In reality, stats & achieves are pretty meaningless. So, if someone has cheated to have their stats say they've killed 9 bajillion monsters, does it really matter? Who cares how long person X has ridden in a minecart for? Stats & achievements should only really have any value to the person who earnt them.