Probably because for most of your life you've been taught, and lived through the idea of limited bed space. I would assume even on a subconscious level, most people know roughly how much room they have to move when asleep, and probably don't roll that far.
That, or you don't roll over and keep rolling, because you're asleep and will probably only turn a bit for minor adjustments.

Thank you very much mum and I both enjoyed your answer very much! Want to be friends? :)

Mum says your elbows stop you from rolling too far anyway.

When you're actually asleep, as in Stage 3-4 you're body doesn't move much as your brain tells your muscle to almost be in a sort of paralysed state. It's the same in REM sleep as well, your brain is aware even if you aren't consciously aware of it yourself that falling is a potential danger and so it takes this into consideration and sends signals to your body to stop moving.