Glad you asked this question as I had wondered the same thing.

So, if BPs don't need exercise and hide almost motionless 99% of the time, why do they need a home bigger than the size of their curled up selves, their water dish, and hides, i.e. at least the length of the snake?

No they are cold blooded so exercise does no benefit to them
What does one have to do with the other?