Maybe he doesn’t want to play football. The “surely footballers want/need to be playing every week” may not apply to him.
Had his well publicised issues at Barca, comes here, seems to settle, gets a terrible injury, and let’s be honest doesn’t get the best reception any more (not without reason - he has been awful).
Pointless and a bit discourteous speculating on mental health etc, but entirely possible that the thought of moving himself and his family to another country and having to ingratiate himself with new teammates, fans and prove himself could fill the bloke with dread. Probably very comfortable turning up for training and going home. He might even prefer having a niggling injury so he’s not even in contention, who knows. If that’s the case, he’d probably stay even if he was on <£10k.
That’s all total conjecture, but I’m firmly in the camp of this being entirely of our own doing. His salary is now a totally sunk cost. Two facts are that he is apparently a thoroughly nice and very generous charitable bloke, and that his work on the pitch has been miles off what was expected and paid for.