Because if you search Firefox and see a badge that says verified, you can be confident that it was Mozilla that packaged it and added it to FlatHub as opposed to some random scammer.
Verification doesnt help at all if the source is not trusted. All this says is "upstream developers maintain this package". Unofficial packages can be safe too, like VLC.
It does help prevent actual malware from being downloaded, though, since upstream developers probably won't publish malware on Flathub.
But this is still a half-measure. I don't understand why Red Hat and Canonical don't treat this issue seriously; people on Linux are used to assuming software installed from the repos are safe, and yet Snap and Flatpak are being pushed more and more despite their main repositories being potentially unsafe.
I can't find it now, but I read that the verification process also includes human review (for the initial verification, not every update), so it should actually prevent "verified" malware (though it does nothing against unverified malware).
Because both Red Hat and Canonical are of the "pay us to care" mindset. If you aren't paying for support, you're a freeloader and need to do your own research.
That's not entirely true with Red Hat. There's a lot of work that they've done in the open source community that they haven't shared back. And canonical seems to think this is a good idea.