Likewise, the same cat who walked across the bed unnoticed a moment before can suddenly seem to put an unnatural amount of weight on your bladder as they stand on your stomach, asking for breakfast. Most cats only shift their weight a little. In all things the cat seeks balance. Humans have to work hard to achieve the same sort of effects. Generally only those who have devoted years to the study of the martial arts have any noticable ability to make themselves impossible to shift.
Cats do it as a natural expression of their cat nature.