Just try getting a popular app anywhere else. There are options of course, but they require a working knowledge of APKs and other technical concepts which many casual users may not be familiar with. Even once you get them installed, these apps often don't work without play services, and -- depending on the source -- can be less secure.
I don't use iOS and have only been able to install many popular apps through APKs sourced from other sites like github and apkpure.
Ill admit I don't know much about side loading but I think that only supports my original point that the average user isnt aware of less intuitive alternatives to the play store.
I'm vaguely familiar with fdroid, but thanks for linking. My understanding is that fdroid doesnt support automatic updates, though I might be wrong. I use obtainium instead.
Amazon store is indeed another option. Personally my issue with the Google play store is the sketchiness of the company though, so Amazon isn't a good alternative for people like myself.
My phone can install APKs out of the box. XAPKs need a separate installer though, but that is a one time problem. Many APKs even get recognised by the playstore which made it easier to update it.
My device either isn't verified or in some way geolocked for some apps so I had to get APKs of them to use those apps and I never had a problem with that.