Because he is stupid as fuck and above all that he ain't good enough. It's clear he won't make it. Claims that he isn't playing because Koeman hates him are absurd. When someone is good enough he will get his chance. Just look at Fati, Pedri, Araujo, Dest or even Mingueza. All youngsters except him are getting their minutes. If he was any good he would play, simple as that. I am willing to bet that he wouldn't regularly play for any stronger team in La Liga. Probably none of the TOP 10 teams would seriously rely on him so that might be the reason why he prefers to sit on the stands.
I hope I'm wrong but I seriously doubt it. He will probably end up like Halilovic or Deulofeu. There is always that one "generational talent" that never lives up to the hype and ends up playing for a mid or bottom table teams.