They probably just want to fight abuse/spam/fraud/scams that are on the App Store/Play store so that it would stop devaluing their brand. "ChatGPT charged my card $299 without permission, I am going to the press!" is a disaster waiting to happen with these scammy apps.
It is different from developers who are using the ChatGPT API to build apps.
Also, the signup process is far from easy. People here often cry about how "difficult" it is to sign up for Mastodon or Matrix etc and that is why the mainstream don't use it. Yet this is the same :)
source: I used to do tech support for my family
My understanding is that this might have been what Apple originally planned to do. However, the popularity of the App Store, IAP revenue, and the focus on services disincentivized them from pushing this route further.
That said, it's possible to package progressive web apps (PWAs) for listing in app stores: PWABuilder[0].
Disclaimer: I've worked on PWABuilder.