> has almost no impact on the average user security

I don't think that's true.

Google publishes malware stats on the on-play and off-play ecosystems and the malware rates differ. Famous examples like malicious versions of FortNite make news headlines. Play policies also enforce various security-relevant action like updating to near-recent targetSdkVersions and fixing a variety of automatically detectable security vulns. No such enforcement exists in the broader off-play ecosystem so it will be easier to install an app with trivial security flaws.

Whether this is a justification for banning sideloading completely? That's another question.

It's not happening as much with Android because Google has a vested interest in making it easy for people to be tracked, so it has fewer restrictions on doing so. The "Ask not to track me across apps" feature on Apple is what this is mainly about for Facebook.

But yes, there have been such cases. Epic successfully forced a massive number of Android users to sideload Fortnite to get around Google's policies, and Google wasn't happy and tried to undermine that attempt.