Playing is fun, sitting on the bench is not. However, if you are being coached, you can still learn a lot during practice. What you learn may come in handy in the future. Leave the game if you are no longer interested in playing or you do not think you will ever be good enough, not because you don't get into a game.
Soccer is a team game. Coaches may pick players based on what he needs. For example, if he needs more defense, he may not put in as many forwards or if he needs scoring or playmaking then he may pick players who are good at those skills.