At 80 years old, he's not ready to retire. He wants to serve until he's 88, when he can use senility as an excuse.
He likes his cushy DC office where he gets lots of attention, the only place the stupid people and liars can have a "successful" 40 year career.
Specter is not a traitor, he's leaving the GOP because the movers and shakers of the GOP in his home state won't support him, so he went a party that welcomed him with open arms.