I think the issue is that premier league teams have massive squads and if your not first choice you dont get game time, and they dont have the time to develop their own players in the first team.
So loans are often the way, and after playing first team football on loan, the player doesnt want to go back in the stiffs.
Hence they move on . (Like O'hare did)
And if Gyokeres wasnt ready for the premier league last year, he sure as hell will be next year ! (Hopefully with us )