Aluminum is softer than steel, so the mold wears out quicker—the models lose details.

Ah cheers that makes sense. One thing I was wondering would they use stainless steel for moulds, to prevent them rusting?

Nope, "real" molds are made out of tool steel. Corrosion isn't really a problem since machines are typically stored indoors.

and it's much cheaper to work. If you're only doing a couple thousand moldings, you don't care as much about mold life