Because people treat it like it's an indispensable town square.
This place is chock full of idealists and techies, it's easy to lose track of exactly how much sheer "inertia" and habit shape people's tech choices. And that's assuming there is a choice at all, maybe you either Facebook up or junior isn't going to any activities, nobody knows what's up in the neighborhood, people will either be harder to contact or (partly understandably) look at you like an asocial weirdo, etc
Cats can be pretty enigmatic and have a lot of individual differences in personality. At least size can't be the whole picture, based on tiny tiny kittens squeaking terror into the hearts of giant four-ton beastly hounds, bears, what have you.
Maybe we humans are just size queens, as nothing stops a chihuahua or something from doing the same.
Some people have suggested that cats get to see you as a "pack member" - dead birds and mice and stuff are gifts because you're just too incompetent at this hunting stuff, so they graciously help you out.