Some of them are just inescapable boxes; some of them look like jails; some are combinations of parkour and slow-blocks ("soulsand" and cobwebs, for example) so that the player can escape, it just takes skill and time to do so.
Because players can build them, but most other games need the "owner" to change the rules of the game. Gosh democracy, freedom, openness ... It's almost as if we just had an election:-)
Some of them are just inescapable boxes; some of them look like jails; some are combinations of parkour and slow-blocks ("soulsand" and cobwebs, for example) so that the player can escape, it just takes skill and time to do so.