Not to hijack, but… there’s a Canadian show called ‘Untalkative Bunny’ that has a similar kind of unexplainable appeal. (It’s mellow and quiet, though, while Spongebob is a little too frantic for my tastes.) The main characters are all animals, none of whom can speak. Bunny in particular is very emotive and expressive, even without words and even considering how badly-drawn he is. There are a number of short skits per half-hour show, with plotlines ranging from the overly simplistic (Bunny frightens himself while listening to a scary radio play in bed) to the surreal (Bunny goes surfing, falls off his surfboard, and ends up visiting an underwater kingdom of sea slugs).
Again, it loses something if you try to read too much into it; I think this kind of cartoon is appealing to grownups because it reminds them of when they were kids, and could watch TV without thinking too much. This one in particular is very unstressful and soothing-- I suspect Spongebob appeals to the same audience, but a different part of their brains.