Well, it’s a franchise. The first one did well enough, and they have another character they can use in other movies. So a sequel it is.
It may also be a matter of wanting to keep broadening the genre. Ant-Man is their heist/comedy vehicle, just like GotG is their space flick and Spider-Man their high-school movie.
Personally, I liked Ant-Man a lot and am completely fine with sequelisation.