Gum disease is very common in dogs. Most people don't know and/or care to treat it.
"doggie dentists" are the vet techs at the vet. They take special traing and get certified to do it. Any half decent vet will offer a teeth cleaning at most visits BUT people usually don't take their dog to they vet nearly as often as recommended. And when the vet does offer it most people don't want to pay for it.
Dogs do get cavities and have teeth/lose teeth pulled more often than you think.