Playing time is not the only means to improve, but it is the most important means.
Every NBA player practices countless hours, but those hours mean jack if you don't get a reasonable shot at showing it in an actual game. Snyder has stifled Exum's improvement by not playing him much, and even when he did allow Exum to play, half the time he was designated to stand in the corner and watch Hill or Mack turn the ball over.
If Exum had been drafted to the Spurs, instead of sitting on the bench watching Chris Paul school every Jazz player, people would be talking about how great a player Exum is and how bright his future was. On the other hand, if Kawhi Leonard had been drafted to the Jazz and played for Snyder, he would be playing at a similar level to what Hood is playing at right now.