Carl Barks was actually prohibited from portraying humans in comics. Western Publishing (the arm of Disney that printed Disney comics) had a very clear code about the Disney comics like Donald Duck, Mickey Mouse, etcetera.

In Donald Duck "Dangerous Disguise", the main protaganist is a human female spy and their are lots of humans (without dog noses) present. It was one of the few that escaped the censors:

The reason was that the Disney world is a separate and distinct place from the human world. In very early forties issues Donald and his nephews hailed from Burbank, California and later he was forced to change it to Duckburg, Calisota (California and Minnesota?). That permitted him more latitude with the 'weather' stories as well. Snow six feet deep doesn't seem like Burbank. ;-)

Western had a whole sheet of no-nos which including putting any character in mortal danger. That one was often flirted with, implying demise, but they probably didn't want to get the bad rap other comics were getting and you will not see many with a gun being pointed at a Disney characters head. This is a far cry from the Warner Brothers cartoon stuff where mortal danger is almost a staple in every scene.

The desire for separation between the cartoon world is understandable, as it keeps the fantasy element engaged and allows for suspension of disbelief. If you look at the page I linked to, seeing talking ducks in a group of humans is a bit bizarre. That's why the dog noses (normal joes), pig faces (rich people), owl faces (judges) and wolves (lawyers and con men) were used as it helps the Disney characters fit in.

I suspect this same motivation was used across the cartoon industry. Humans and talking animals are a bizarre combination and those cartoons that use both often seem disturbing and distasteful.

If you look at the symbology of many cartoons, cats are generally evil. Mice are good. Birds are good too. The smaller the animal the more likely it is to be good, the larger the animal the more likely it is to be bad. The psychological appeal is to children and adults with complexes. The "little guy" always wins in the cartoon. The big guy loses. Kids subconsciously identify with the smaller animal and persecution complex adults also identify and enjoy seeing the little guy defeat the big guy.

Even Bugs Bunny, who is already shorter than his human enemies is only bested by the little mouse type characters. The unspoken rule is the "little guy" always wins.

They didn't have think tanks in those days, though the term "brain trust" had already been coined. What they did have were paid psychologist consultants who helped steer the industry once it started raking in big bucks.

Basically, the entertainment industry is all about providing escape mechanisms for folks who need them. Appealing to the lowest common denominator, using subtle psychological twists that comfort their viewers, is the path to the "millions of clams".