Because he is. He’s one of the top-two villains that the Avengers have that are exclusively theirs, as opposed to being a Thor villain or an Iron Man villain or a Cap villain. Ultron is the other one. Apart from that, they’ve mostly fought groups, the Masters of Evil and the Lethal Legion and so forth. But Kang and Ultron are the big two, and so every time they’re featured in an Avengers story, it should be an event.