It's because the only accepted way to distribute software for Android is through the Google Play store, outside of China. Many companies even distribute their

It's because the only accepted way to distribute software for Android is through the Google Play store, outside of China. Many companies even distribute their internal tools over the Play store. You can't expect normal users to go find some arcane setting and accept security warnings in order to enable standalone APK installation. Incidentally, the option is hidden in completely different submenus depending on Android version and customizations by the manufacturer. The developer agreement for the Play store even comes with an "anti-competition clause," explicitly forbidding anyone from distributing software that could compete with Google's app distribution business. Unless you do it outside the Play store, in which case you have yourself a bit of a chicken-and-egg problem.