Plenty of players improve and dont get drafted higher.
Take UCLA dudes like Jaylen Hands and Kris Wilkes. They both went back, got better, and still arent first round picks. Each of them lost career earnings not leaving school. Your chances of becoming a first round pick dont improve by staying in school.
neither were first round picks last year but at least bet on themselves and showed some sophomroe improvement. ucla (and the pac-12 in general) was also a dumpster fire this year which didn't help. they lost maybe 1 year of g-league salary; going back to school was worth the risk.