Sancho is not taking his football career seriously and doesn't want to improve his performance so that clubs can pick interest in him. He is after being paid heavily, and he forgot to understand that there's time for everything. United wasted a lot of money on paying Sancho and the rest for their inconsistent performance. This has always make United to put him on loan to other clubs every season. If Sancho is sent back to United, it will be a burden on the club.
Indeed, Sancho's performance has declined greatly and I don't think he is not serious about being able to perform at his top performance, but it is not easy for him to get back after he slumped at Man United.
Now several teams that want to recruit Sancho are indeed constrained because Sancho wants a big salary, and indeed that is also Sancho's fault I think because if he continues to be selfish then no top team will want to give him the opportunity to develop.