User installable 3rd party mobile app stores cannot implement automatic upgrades, background installation of apps, or batch installs of apps like the Play Store can. These limitations are designed by Google and are implemented in Android.
If the user tries to install an app on their own, they're shown scary warnings and must adjust arcane settings, but if they use Google's Play Store, no scary warnings are shown and no settings need to be adjusted. They're told they're "protected" by Play Protect, but aren't shown scary warnings about the fact that the Play Store is the main distribution method for malware on Android[1] when they go to install apps with it.
The Play Store isn't alone in being a vector for malware, as Apple's App Store is responsible for nearly half of a billion malware installs of just XcodeGhost alone[2].
Like Apple, Google also mandates that apps distributed via the Play Store must use Google's payment system[3], and give Google a 15% to 30% cut of revenue:
> Developers charging for apps and downloads from Google Play must use Google Play's billing system as the method of payment.
[1] https://www.zdnet.com/article/play-store-identified-as-main-...
[2] https://www.vice.com/en/article/n7bbmz/the-fortnite-trial-is...
[3] https://support.google.com/googleplay/android-developer/answ...