Why are 50% purebred 50% purebred mixes considered mutts?

Such as a lassapoo or maltipoo if it is a 50% purepred 50% purebred why is it considered a mutt. Isn’t mixing two purebred dogs then breeding their puppies for several generations how you get new breeds? When I said 50% purebred 50% purebred mix I meant mixing two dogs that are 100% purebred.