I think Google wanted to make things easier for them, not wanting to be blamed for or having to deal with dozens of third-party apps' issues when called by any app that uses the camera for some purpose.
LineageOS sadly doesn't change that, either. But LineageOS, as @Hook already reported, does at least allow us to configure a standard app for the dedicated camera button. (Was already true for LOS 18.1, by the way.)
I'm wonder, though, whether it also might have been possible for Fxtec to add such a setting for stock Android. Perhaps someone who's helping them with the stock ROM in th