Play store allows basically anything (including obviously fake/scam/adware/spyware apps), there's nothing to be gained by doing this on Android, you'd lose an easy distribution mechanism that doesn't limit you, helps you gain victims and provides credibility - it's on top of Google then it must be good, right?

Also consider the market share of iOS and Android. Why bother for such small amount of price sensitive users?

iOS is another story entirely. The App Store has a lot of demands on the app developers wrt. UX, compatibility, etc; there's a lot of money to be saved (and faster time to market gained) by not publishing apps there. And then you can also bypass the security/privacy screening.