There are probably many reasons why monks live in monasteries, but I’m interested in just a couple in particular. It’s this idea that once you reach a heightened level of understanding you realize the futility of seeking out people for enlightenment. It is far easier and more effective to let people come to you because their minds are open and ready to learn. Imagine all the skeptical, close-minded people on the Internet being taught by even the most wise monk. They would be trolling and arguing and claiming they won, all while the monk sat calmly wondering why he even bothered. It’s not worth trying to reach their minds. It’s better to let life flow naturally and if they wake up, they will seek out the information themselves. This is why Buddhism and Zen are so superior to organized religions like Christianity. They do not indoctrinate because they do not care to. Their level of understanding is so high that it all seems petty and futile. Enlightenment is an individual journey that will either happen naturally or not at all.
The next reason results through the elimination of suffering. All personal suffering is derived from desire and when you work to eliminate them, you find yourself in a state of calm. You must understand that, unless you’ve practiced this, you may never have actually experienced life without suffering. Even from a young age, we are grown up in a world where you are constantly wanting things or comparing yourself against others. It is very possible that a life without suffering is not one in which you can even comprehend until it actually happens. So when monks and enlightened individuals do eliminate them and reach this contentment, they no longer need the external world. They may enjoy to wander about, but they do not need it. So monks sit pleasantly and content, up in their monasteries because that is all they need. A nice calm, quiet location where they can practice meditation and live in constant peace and harmony.
People will argue, and I would have in the past, that this behavior is lazy and self-indulgent. That these monks are just wasting their lives and should be out doing things. Or at the very least helping others and teaching their lessons. There are so many things wrong with this argument. First, most people, if not everyone, aren’t in a position where they should be commenting on another person’s life. The vast majority of people are robots, toiling away, completely unaware that they exist (or, more accurately, do not exist). No one is qualified to judge and assess another person’s life, especially when their own is completely out of their control. Second, the universe does not value anything over anything else, meaning that no matter what you do in life, it is equal to any other task or achievement. We only apply subjective value to things; there is no such thing as objective valuation within the properties of reality. Ex. the beautiful women isn’t actually beautiful, she’s just subjectively beautiful. Next, the reward and understanding that comes with enlightenment and meditating in monasteries is complex and abstract. While it may appear lazy and wasteful to sit meditating and reflecting on the universe, it is actually quite taxing and tiresome. The work may not be tangible, but these monks and enlightenment seekers are doing far more than you might imagine. All-in-all this argument lacks even the most basic understanding of what enlightenment is and the more you begin to unravel the mysteries of this life, the more you realize how wise and intelligent it is to simply sit in a monastery.
Update: I’d also like to mention that attaining and maintaining an enlightened state is very difficult and thus remaining within a serene monastery may expedite the process.