I think it’s more about the defaults included, plus Google hides sideloading more with each new version of android. They have a semi-reasonable worry of “If we show off how easy sideloading is or can be, that enables bad faith actors.” Like Microsoft and IE/Edge.

But on a fresh install of Android, if you download f-droid from Chrome or Firefox, you need to:

• Allow the browser to install apk files
• Install the apk
• Then enable f-droid to install apk files

Meanwhile Play Store doesn’t have any of that, it’s enabled and allowed to download and update anything without user prompt, it’s an opt-out.

Plus with how anything that conflicts with a default app like a Clock app, there’s a decent shot it might not work due to the default battery optimizations, the internal syscalls for “Google’s Clock” instead of “Default Clock” it just kinda makes using anything that’s not the one included with your phone more of a headache.

It’s easier with SMS, Dialer, browser, and Home launchers, but if you wanted to change any other important apps like your calendar or email it’s kind of a headache.

It’s a monopoly in the same sense why Disney and Microsoft had to be investigated from their recent acquisitions. Monopolies in the real world aren’t just a “this and only this is allowed”, but also more of a “you have a choice, but do you really”

To add to other replies, play store protect flags apps with same names but different keys. This can cause false positives for security issues that pop up, I believe f-droid signs with keys generated through their system so they don’t match a play store version. Users then could think it’s a virus from f-droid and use play store instead.

I’d guess because Google Play is a set of services that just use “Play” as a prefix.