Because there's no point in complaining that the game is not free when :
- The game is free on Android/iOS, so you could just get it on your phone or use an Android emulator ;
- The game is only 5$. This is absolutely cheap for what it is.
With that in mind, it's quite clear that people that are complaining about the game not being free are either kids, or people who just don't give a fudge about the devs' needs. (and those people may also be kids.)