Purely hypothetical but...
Maybe he gives 100% in training
Maybe he loves the club and really wants to play for us
He did well when he first came in and maybe he's convinced he can still play with the same returns if given a lengthy run
Maybe he sees the revolving door of managers and he's waiting for one to give him a chance
On the outside it might look like he's a mercenary because he's not moving away, but there's always the possibility of any combination of the points above, we really don't know.
Or maybe he's laughing at a company paying him millions a year to sit on his arse... Probably seems more likely after writing this