There’s a good explanation about that here: https://webaim.org/blog/user-agent-string-history/
The issue is that a lot of sites used the user-agent to determine if the browser supported particular features (e.g. show a fancy version of a site if the user is using Netscape, otherwise show a basic version for Mosaic, lynx, etc). New browsers had to pretend to be the old good browsers to get the good versions of sites
This is why getting rid of the user agent is a good thing. Sniffing the UA is a mess.
Apparently MacOS apps can be sandboxed and store data securely such that no other apps can access it, in an encrypted format. I wasn’t aware of this either, but it’s been a while since I’ve used MacOS. It sounds like the ChatGPT app explicitly opted out from this sandboxing model.