It's still something of a mystery how cats have been considered so humorous on the internet for such a long time. Perhaps it's something to do with the mischievous look in their eyes, or simply because a lot of people who make memes happen to have feline companions too. The fact that they are naturally quite entertaining has to have a hand in it also.
However, maybe the most convincing reason of all is because of how versatile they can be. With memes in which other animals take center stage, they're often associated with a certain kind of content — dog-related memes are often more wholesome, for example, and possum memes bring out the inner trash goblin in us all. With cats, meanwhile, it's widely accepted that the memes they bring are going to be a mixed bag. They could be cutesy, or they could be the most feral kind of sh*tpost imaginable. It keeps us on our toes.