[jlbribeiro]

.io are indeed "expensive", but to add one datapoint to your question: there's a concept/genre called "io games", mostly involving browser-based multiplayer games like this one (e.g. Agar.io, Krunker.io, and many others. a quick search leads to several lists of those; some of them don't use an .io domain, but lots of them do). That's probably another reason why, besides the "cool factor"?