The pool may look like it has a lot of players, and your odds of getting a call are slim, but this is not usually the case. Generally speaking there are two kinds of pool players - those that have joined the season after the league filled all teams, and those who do not wish to play full time.
Some members jobs or family commitments would have them miss so many games they choose to play in the pool. It is these same people who often have to turn down offers to play pool games for the exact same reason. So, even though there may be 30 guys in the pool, there may only be 5 or 6 people available on a given evening.
If a member in the Pool is going to be unable to play for an extended period of time (i.e. an injury), they can be assigned to the Inactive Pool until they are ready to play again.