Google used to publish Apps almost instantly, no quality review whatsoever, as result, the Google Play Store was full of garbage. Crappy Apps, malicious Apps and overall useless Apps were rampant. Several scandals and privacy breaches after, Google said, no more… “We will review the Apps and ensure some quality across the board”, (turns out that, although criticized, Apple was right, all the time).
Submissions for mobile apps for iOS are subject to approval by Apple’s App Review team, as outlined in the SDK agreement, for basic reliability testing and other analysis, before being published on the App Store.
Applications may still be distributed ad hoc if they are rejected, by the author manually submitting a request to Apple to license the application to individual iPhones, although Apple may withdraw the ability for authors to do this at a later date.