My experience of Signal is that people don't reliably follow up on safety number changes, yet keep using it in a MITM-vulnerable way. Including myself, honestly, because people change or reset phones frequently. Is your experience different?
At least with GPG I can factory reset all of my computers and phones and not have to re-establish trust if I take the right steps to preserve the secret key information.
Even if I don't preserve that correctly, people change computers less often than phones.
On the other hand, I'll admit that my GPG key is newer than my Signal number (which I've owned for 15 years), due to upgrading crypto algorithms.