HTTPS can be MITMed by anybody with a root certificate (or who is able to dupe the holder of a root certificate). Signatures rely on (at best) multiple hosts or domains not being compromised. You can also mirror and cache distro ISOs when retrieved by HTTP or bittorrent.
In the end, who cares how you downloaded it, so long as you can verify it with a GPG signature?
I've also found some projects where the key used to sign the software isn't even consistent between releases, so you don't even have the (minimal) protection offered by checking the signatures against "known good" builds.
I wonder how hard would it be to socially engineer yourself into trust chain. I haven't read GPG manuals, but I'd be surprised if there were strict protocols followed by everyone when signing others' keys. Most people probably don't or can't insure that there is no mitm when signing keys and can't reliably verify that government id papers aren't forgeries so likely reliable chains of trusts exists only between people that spend large quantities of time together and exchange fingerprints over secure channel.