Don't kid yourself. McNabb is the Eagles QB, provided he's healthy. Garcia has proven himself to be a great QB in his own right, probably capable of taking a team to the Super Bowl.
But Jeff is 36 and isn't interested in going to a team that's a few years away. Most of the teams that are good enough already have a starting QB, and they won't change for Jeff.
Garcia simply wants to use this opportunity to get as much money as he can, while being in a situation that will work for him. Unless some team offers him a starting job and a lot of money, chances are he would prefer to stay with the Eagles.
Which would you choose to go to a team like Detroit to be a starter and get beat up every week behind a line that can't block and no chance to get anywhere, or be a backup for Donavan, hope he doesn't get hurt, and get a Super Bowl ring and the best seat for the Super Bowl?