Sure, there's Fortnite, and a smattering of other apps that aren't in Play Store. But, even though Android has made sideloading easier over time, it's still not that common; most popular apps don't even offer an apk download outside the Play Store. Attempting to load an apk with the setting off used to just give a failure message that quickly disappeared; now it actually takes you to the setting and asks you if you want to do it (with scary text suggesting you don't).

Certainly, Play Store doesn't feel as restrictive as the App Store, so that's probably a factor. But I doubt popular apps will drop Apple's store, because it will significantly reduce the number of people willing and able to install their apps.