My insane peppa pig theory made for laughs.

Many cases of media with anthropomorphic animals have little to no real explaination on why they are human-like in the first place. The most said is usually just what the show is, like zootopia or beastars where it's "what if animals had human intelligence but are animals by heart". For peppa pig it's seemingly just to appeal to kids more or just because, although a few observations I had make me think there is probably a reason why this is the case.

What I mean is, Ben and Holly exist in this universe, whether it be as a fictional book or something else. This theory is based on the latter option, due to the existence of magic in Ben & Holly's Little Kingdom anything can happen which convinces me that Nanny Plum or The Twins (who are the only ones reckless with magic) had cast a spell on the world to turn most of the mammals into people and let them rule over resulting in the actual humans dying from regulat animalistic rage before they settled down to be more human-esque.

Another point to reinforce this is that humans actually do exist in Peppa's world, examples being the Queen and Santa. Former being important enough to protect throughout and the latter being a magic. Human cavemen also used to exist, and there are human pirates, the pirates live on the sea meaning they were out of reach and the cavemen were before this ravage.