I'll preface this with the point that I believe he's happy here and won't do it because he isn't desperate to leave... HOWEVER there's one big reason that players are particularly eager not to hand in transfer requests even if they massively want to leave: Loyalty bonuses, they can be worth millions and are only paid up to the player if they leave without requesting a transfer (or across the length of their contract if they stay). My favourite part of the whole Sterling saga was that the cheeky [Poor language removed] got a nice juicy loyalty bonus from Liverpool when he left even though he'd done everything but hand in a transfer request this summer to force a transfer.
You are right re.loyalty bonuses. they can be millions at times. It's why many people said Rooney didnt hand one in when Moyes went to Utd and Chelsea were linked, his was 4-5m or something crazy