Epic did for a time only distribute their game through their launcher, available only as an APK from their website, but they eventually capitulated to Google's policies and listed Fortnite on the Play Store. They did this because it is evidently not realistically possible for a commercial app to be distributed outside of the Play Store and reach a critical mass of the Android userbase, whatever the cause(s) may be.
Google has made the process of installing alternative app stores obtuse enough that general consumers might not do it. It's not that obtuse, especially to readers of this site, but it looks as though it's enough. There's also the restriction that app installs and updates cannot be done in the background.
Another possibility, or perhaps in addition to the above, is that the general public never goes outside of default app stores to search for games, or is never persuaded to do so. It's the path of least resistance to only use the one(s) included with the phone. Fortnite is the one attempt at a massive enough scale that could have broken this pattern, but it didn't, even though they had Fortnite on all the other platforms to use as an ad, which targets exactly the people most likely to jump through the hoops.