Because Rondo is a Celtic, and because it's not really up to him (at least while he's under contract).
He comes across to me as a pretty loyal guy, and a VERY particular guy. If he likes his boys on the Celts, and the environment and tradition of the team, he'll want to stay.
I don't envision that much of a rebuild anyways. We've got good young players and PP and KG will be here next year to win a championship. So the year after, you got Jeff averaging 22, Rondo 17/11, Sully 14/9, Avery 12, Lee 12, Fab Melo 6/6 .. . . Celts will still be a .400 or .500 team. T-Will will probably by back up Center if Doc is still coaching.