I'm not saying I have evidence that this happened to PGP specifically, just that it doesn't seem at all implausible. If the CIA told me my code was never to get too easy to use, but otherwise I could live a long and happy life and maybe a couple of government contracts it would be hard to argue.
Why a mass-market interface never took off (GPG and other descendants notwithstanding) may indicate that the whole cryptographic idea is inherently not amenable to user-friendliness, but I don't find that hypothesis as compelling.
(It could also be an unlikely coincidence that there's a good solution not found for lack of looking, but that's even less plausible to me.)
signify[1] is approachable at least for the power users - I could print out that man page on a T-shirt. HTTPS is ubiquitous and easy, thanks to ACME & Let's Encrypt. E2EE with optional identity verification is offered in mainstream chat apps.
And of course there are usability improvements to GPG, being made by third parties: Debian introduced package verification a couple decades ago, Github does commit verification, etc. What's to stop e.g. Nautilus or Dolphin from introducing similar features?
I wonder why there aren't more, but there are some, for example Proton's efforts towards encrypted email.
https://proton.me/support/how-to-use-pgp
(I won't mention the relative shortcomings of HTTPS and E2E chat apps here.)