Games are really easy to test viability for, though. The game has perfect information about itself, rarely connects in a complex way to anything outside the game, and it's easy to replicate the user's context just by sitting in a dark room on your own. The test is simple - is it fun?

By contrast, most apps need to connect to people, software and things outside themselves - they're part of complex systems, and it's really hard to know if they work well without putting them into use and seeing how they perform - and that means selling and talking to users, not something you can do in isolation.