Terra (Terraria Version)

Terra
Terra is where Terraria takes place. A lot of this stuff here is pure spectulation, so just give it a shot.

Terra is a land of mystery. The character you  play as is a person who figured out how to go to alternate dimensions via a device located in their pants, which is why you can't take off your starting pants in Terraria, unless you replace it with armor. Without using the device the character cannot go past the walls of the dimension they are in unless they use the device AKA the start menu.

Some dimensions are bigger and others are smaller.

But the reason you are dimension warping is to defeat all evil, which explains why the game never ends. It has an endless supply.