First off, unless these players hsbaseballfan mentions had very good reasons (and I have zero knowledge of their situations), they didn't "leave their high school teams". They quit. Plain and simple.
We always had a rule in my family- you didn't have to play any sport you didn't want to. But if you did play (i.e. join/make the team) under no circumstances were you to quit. Of course, there can be exceptions for extreme circumstances, but for the most part you were going to ride it out. It was never an issue.
IMO quitting for the typical reasons (I never get to play, coach doesn't like me, I have something else I'd rather do, etc.) is selfish and unacceptable. Plus, as far as I'm concerned (and as far as my coaches were concerned) if you quit a team once, you were quitting that team forever. Quit baseball as a 9th grader? Save your time the next three years. Again, there are exceptions, but those exceptions will probably follow: 1) a one on one sit down with the coach for the player to own up to quitting and ensure that's not going to happen again and 2) a summer/offseason of that player busting their tail to show the coach that baseball and this team are important to him.
Now, if a kid just didn't play as a 9th grader and then tried out later in high school, that's a different story. I'm differentiating here b/w making the team and quitting vs. never trying out.