Because nobody uses it except a handful of uber-nerds who can put up with wasting hours of time for "altruism".
The answer to freedom isn't another marketplace. It's legislation that requires Google and Apple to allow web installs without scare walls or deeply hidden permission toggles.
You should be able to just download an app. And Grandma should be able to do it.
There's nothing insecure about it in a world of sandboxing, permissions, scanning, blacklisting, etc. When the tech giants are more than happy to sell ads to malware, you know their "safety" stance is just economic moat protectionism.
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Edit because of rate limits:
This is not derogatory. This is making the point that the wrong battle is being fought. F-droid fans are cheering a small victory, yet losing the entire war.
F-droid does not matter in the grand scheme of things. It's not the point. The point is the app store monopoly. That there are only two platforms and that both are almost completely locked down.
F-droid users don't even see that Google is purposefully allowing this as an antitrust sponge in their calculus. Google doesn't mind because 0.0001% of their users will install this. If they had a sense that the regulators wouldn't bother them, they'd just as well close this loophole. And they probably will eventually.
This isn't beachhead, and even if it was, it isn't sound footing or a stable foundation. It can easily go away.