Why isn't Pluto dog an anthropomorphic character while other ANIMAL characters around him all are?!?
Pluto was never more than Mickey's pet. He appeared in about 1930, and he even spoke 2 or 3 times before it was decided he didn't speak. He's sort of like Harpo Marx or Dopey Dwarf, he expresses himself through expression and pantomime. But you're totally right, all the background characters in early Mickey Mouse cartoons were animals, mostly dogs. His nemesis, Pegleg Pete, was also a dog. Fleischer studios also had animal casts, in fact the first couple of Popeye cartoons had animals. Did you know Betty Boop started out as a dog? Goofy is actually a relatively late
Likely, it is due to the fact it would be really weird for Mickey to be leading an anthropomorphic dog around on a leash, keeping him outside in a dog house, or just by keeping him as a pet in general. The one that always got to me more so than this was the old "Quickdraw McGraw" cartoons: he's an anthropomorphic cowboy horse who rides a regular horse! They're all just cartoons anyway - don't overthink them.