Absolutely. And how many merchants, both online or in person that you have spent bitcoin with, use PGP? For me, the answer is zero. Even getting peer to peer traders to use PGP is a challenge.

And regardless, even if everyone did use PGP all the time, that's entirely separate to bitcoin itself. If you want to use PGP as a solution, then there is nothing stopping me from also applying the same solution to Monero, for example. You give me your Monero address via PGP, and I can release your address along with the other necessary information to prove I paid you: https://www.getmonero.org/resources/user-guides/prove-payment.html