Because of the risks associated with mating and pregnancy Farmers generally try to prevent ‘unnecessary’ mating. While I’m sure the mule wouldn’t consider it unnecessary, farmers generally want to restrict breeding opportunities to their chosen ‘studs’ (the males with the best stats essentially).

There is always a slight risk of injury during mating, and a pretty high risk during pregnancy. These are very valuable animals and even if you didn’t care about their well-being you would want to restrict mating and pregnancy to only be happening under optimal conditions.

Letting a mule try to get another animal pregnant to see if they are fertile or not just isn’t really worth it when raising livestock.