Drywall mud, also known as joint compound, was created to bond only with paper drywall, and to be used as a drywall finishing product. When you look at large walls and wall corners, it may look like the walls are all one piece. What has happened is that the drywall contractor has used drywall mud to create a seamless looking wall.
Drywall mud cannot stick to plaster or painted walls. Spackle is designed to be used as a repair product on painted or plaster walls. It can be applied, and then sanded after it has dried to be painted. Drywall mud is not commonly used as a repair compound.