Sessler is too big for Giantbomb, he probably wants to start his own site or company... I'd assume he makes a bit more than normal journalist being a TV personality so they probably wouldn't be able to afford him.
He's a big enough personality that he can pull together the resources to do his own thing (and probably will).
I'd love to see him team up with Giantbomb/Gamespot. He has a history with a lot of these guys, they have the resources/backing, and will allow him the room for him to do his own thing.