Thing with the wild is that they'll be able to find a nice enclosed space to hide in, and they'll stay in there to feel safe. The wild is full of tight hiding spaces for them to go into, so they don't have far to go when they're looking for another place to hide.

In a tank (or whatever you're putting it in) there are only two hides for it to go in, and it has lots of space that it has to travel to get to it's water or to get to the other hide and that'll seriously stress it out because it feels completely exposed just to be able to get to the hot side of the tank.