I think it's because as we learn in DMC3, Mundus in DMC1 is dead, or so we think....? Mundus in DMC1 was using vessels of bodies or characters in order to show off to Dante that he has the power, and by the end of the game, he creates a dimensional portal to fight the son of a Sparda.
There is and there are a lot we don't know about Mundus and his status going forward. So, in DMC5, just assume that Mundus is alive.