Goofy and Pluto are both dogs that exist in the same universe, yet one walks, wears clothes, and talks; the other does none of these things.
It seems that in the Mickey Mouse universe, they have bred a slave caste of dogs that they keep naked, collared, illiterate and intellectually/developmentally stunted in order to bring sick pleasure to their masters; and the higher caste of dogs literally does nothing to end this practice—in fact, they seem to accept it as totally fine and normal.
It may be that mice are the superior life form in their world, and there’s an implicit threat hanging over Goofy that if he steps out of line, he too could end up like Pluto.