User blog comment:Master Sima Yi/Assassinews 02-03-'12 — Leaked AC3 info!/@comment-3979092-20120303000936/@comment-1888015-20120303010803

Abstergo had obtained the Piece of Eden prior to having Desmond as Subject 17. It's stated in an email, which is why they needed to locate another.

Also, considering that Ezio already obtained the seals and gave them over to the Ottoman Assassins in the novel, there'd be nothing left to travel to Masyaf for.