android allows is but its hidden away behind a setting and discouraged. in practice it is almost the same system, very few people have a big enough product that they can make money sideloading it. fortnite may be the only example
I'm sure the failure of distributing Fortnite independently will be a strong example against Google when the anti-trust lawsuit will inevitably come.
>“After 18 months of operating Fortnite on Android outside of the Google Play Store, we’ve come to a basic realization,” reads Epic’s statement. “Google puts software downloadable outside of Google Play at a disadvantage, through technical and business measures such as scary, repetitive security pop-ups for downloaded and updated software, restrictive manufacturer and carrier agreements and dealings, Google public relations characterizing third party software sources as malware, and new efforts such as Google Play Protect to outright block software obtained outside the Google Play store.”
https://www.theverge.com/2020/4/21/21229943/epic-games-fortn...
The hard part is the baked in dependency on Google Play services in most apps.