Why do monks live together?

People choose to become monks and nuns because they wish to dedicate their lives absolutely to God and the the work of God (or indeed to the work of their respective religion - be it Buddhism or Hinduism). In choosing to do this, they put no thing and no one before their faith, and are absolute. They seek God by living pure lives; following the example of prophets before them (and Christian monks follow the example of Jesus, who is not believed a prophet), aiming to become more like God, and to go to Heaven.

They become monks and nuns to be separate from society, to escape The Temptations of society and the world, and to seek sanctity, living a Godly life. You would not become a monk or nun for the money, as you may not receive any and will live off natural products grown by yourself under the grace of God, but yet you gain greater riches than you could deem imaginable in the Kingdom of God in Heaven, and on Earth you live a good and satisfying life too.