"He has been offered contracts to get back in and turned them all down"
Uhm... no, he hasn't been. He's not been offered a single contract.
The Seahawks were, I believe, the only team to bring him in for a look.
Yes, he moved his combine at the last minute after objecting to some of the conditions. There were 7 teams at the revised combine, and none were interested in him.
He and his supporters have claimed it's collusion on the part of the league and the owners. Given that the NFL settled with him over his lawsuit on that front, there may have been something to that... or there may have been nothing at all and it was settled just to keep it from being another huge Kaperdistraction.
He wasn't a particularly good quarterback during his time in San Francisco, and it's likely that the 49ers would have been cutting him even had he not opted out of the last year of his contract.
He had one good year -- 2013 -- and every year after that was mediocre, at best.
Even with mediocre numbers he should have been able to easily find at least a back up position in the league. That he never has speaks volumes about teams' willingness to put up with the monumental amounts of distraction he provides.