Simple...JV is a tool to develop varsity players in larger school programs. The nature and number of JV teams depends on the size of the program. At some schools, you might see two or more teams below varsity level. You don't see seniors on JV teams because if they aren't good enough by that time for the varsity team, it's time to find a new activity.
For many, and perhaps most, school ball is a maddening experience. Unlike TB, you don't get to select the coaches or team mates. You get what you get, and even in better programs, the "sausage making" is often somewhat messy. I've also noted something of a "Lord of the Flies" mentality among some players in school ball, which would be squashed on any good TB team in a matter of seconds.
For many, and perhaps most, school ball is a maddening experience. Unlike TB, you don't get to select the coaches or team mates. You get what you get, and even in better programs, the "sausage making" is often somewhat messy. I've also noted something of a "Lord of the Flies" mentality among some players in school ball, which would be squashed on any good TB team in a matter of seconds.