Presumably part of the reason he stayed with the beeb is because it offers opportunities that other broadcasters can't offer, such as radio and Eurovision
By the looks of things Graham has a perfectly comfortable standard of living doing jobs he enjoys at a broadcaster which highly rates him.
If Graham wanted to combine TV and radio it sounds like there's quite a few commercial stations desperate to bag big name BBC talent at the moment, and ITV would surely be interested.
But there have been so many examples over the decades of people moving from the BBC to ITV and hugely failing. And in terms of radio, had Chris Moyles hung around the BBC he'd probably have Sara Cox's Radio 2 show by now, instead of being on a minor digital station.