Yep, Debian, Apple, F-Droid all sign binaries themselves. The posters here mostly mistakenly believe that they can fight against the owner of their operating system without replacing the whole thing.
In the case of Apple, I agree. It doesn't make sense to use their ecosystem without trusting them fully.
However, in the case of Android / Google Play, right now the developers have the option to choose between APKs and App Bundles. Why take away that choice? If App Bundles are superior, then developers will choose that option freely. (Google Play is already really pushy when recommending to use App Bundles.)
I think this is exactly the bit people don't like. I don't want them to "own" my operating system.