I agree with interpol. There are always new info.plist privacy settings and the like that apps need to support. As I recall there were also some relatively recent TLS and https security requirements. Then there are things like dark mode support and... nevermind.
I'd turn the question around. Why wouldn't an Apple customer who buys a product expect it to be up to date, well-maintained, and reflect current best practices in security and privacy?
Saying that it's not convenient for the developer isn't a great answer to that question.
They do maintain compatibility, it's just obviously running in compatibility mode because of hardware changes.
If your app doesn't account for a notched display, or a retina screen, or multi-tasking or whatever, then there's nothing Apple can do aside from run it as it used to run on older hardware, without supporting the new features.
I'd turn the question around. Why wouldn't an Apple customer who buys a product expect it to be up to date, well-maintained, and reflect current best practices in security and privacy?
Saying that it's not convenient for the developer isn't a great answer to that question.