Well, today, we're just dealing with "why Pluto doesn't."
Pluto's first appearance precedes Goofy's by two years. Pluto first appeared in "The Chain Gang," a cartoon short dated September 5, 1930. He was unnamed and homeless, before officially becoming Mickey Mouse's loyal pet. He was first referred to as Pluto in a 1931 'toon called, "The Moose Hunt."
Now to the question at hand: if there are to be two dogs, why does one get to possess human qualities while the other prances wordlessly around, panting and collared?
Well, we're guessing that, because Pluto was first to appear, his arrival answered a writer's desire to give Mickey a household pet. That he happens to be a dog and the clothed, speaking, two-legged Goofy is as well could be coincidence.
It's definitely certain that Pluto's value has never been diminished by his pet status. He's been given his own storylines, family members. (His wife is Minnie's dog, Fifi the Peki. Together, they have five pups--only one of which is named), and foes. He's also an Academy Award winner for 1941's Lend a Paw.
So don't feel bad for Pluto's poor paws, all of which are in constant use. He's come a long way from those early comic strip days as a stray. He may not be able to deliver a worded punchline, but he's been plucky enough to firmly plant himself within our pop culture posterity.