Don't know about the size of city/town or student population at the high schools that are refered to in this thread.
My own personal experience at our high school that has been over crowded waiting for a new school to be built, has been this:
At our school we have three teams.
Freshman, JV (exclusively Sophmores with the occasional Frosh player - no Juniors) Varsity
(Juniors/Seniors/occasional underclassman).
Back in my day Juniors could also be on the JV team. Not here, so if it just so happens that the varisty in any particular year is heavy with talented Seniors - there is simply no place for some of the Juniors to play until the next year when the Seniors graduate and roster spots become available.
On the one hand coaches might say they want to see them out there trying out for the team, but in reality those that do are treated pretty badly. I am not talking about slackers, these are talented guys that are victims of circumstance. They don't sit home playing video games while waiting for their turn, they usually find another sport to play either at school or outside the high school.
I also think some kids these days get a sense of burn out, and find out their lives have been so planned and rushed by (parents) trying to keep up with the next guy, that they get to be 16 or so and discover that their interests are broadening. I sat next to a young man once at a tournament and we began to chit chat. He told me he had never been allowed to skateboard,water ski, wakeboard, snow ski, snowboard, ride an ATV or a motorcycle, or even a BMX bike. All because his parents felt he would get injured. It took me by surprise that such a young man felt like part of his life had passed him by and I could tell he was more than a little miffed about it. This is not to say that he has many more years to experience these things, of course that is true.
As the parent of a college freshman baseball player, I can tell you this. Everything he did in high school sports was left behind the day he graduated, his accomplishments, awards and accolades, along with the politics.
And yes, my student athlete was a starter at each level, he was not one that had to make the choice to sit out JR. year, he was in the minority.
There are so many different situations that may apply to a kid not going out for a sport, I think it would be wise for us to take this into account, and not judge these guys (or their parents) too harshly.