Perhaps it's a matter of priorities. Given how much more powerful today's iPhones are compared to those from 2009, Apple could run older apps in an emulator.
Here's a data point: I am demanding it. I purchased software and I'd like it to continue to work.
The Ace Attorney collection was excellently packaged for iOS, and I want to keep playing one of the best-published versions of this trilogy, but now the UI doesn't render properly.
I'm not saying that it's trivial, but we both know it is absolutely possible. Apple keeps putting the focus on "gaming" in its iPhone keynotes, but if they were serious about gaming, they'd make the games I already bought continue to work.
It's not an OS feature because Apple has an aversion to maintaining any kind of compatibility layer for more than a few years, it's not a third party app because Apple doesn't allow JIT code (eg. high performance emulators) in third party applications on iOS.
Demand isn't the only reason to make a feature, sometimes customers might want something only after they're given the opportunity to recognize it's value.
Designing an app to emulate iOS games would be extemely difficult, let alone allowed on the app store. Backwards compatibility requires deep forethought and careful engineering as well as draconian enforcement. I think the technical difficulty plays a huge role here, when weighed against Apple's culture (sleek, minimalist) and the impact it would have on profit.
A modern iPhone sure but what about the iPhone 2/3G? That’s basically a low power ARM chip and we have Nintendo Switch emulators with very good performance. Anything pre-security chip should not be that difficult to emulate.
My guess is that while it may not be too much effort to get a mostly accurate emulator that works well enough for hobbyist use, it'd be a lot of effort to get something up to the compatibility and usability standards of an official product.
Many older apps may use undocumented functionality or non obvious quirks of the system that an emulator may miss, which means you'd need to have a QA team testing individual apps for compatibility.
Part of Apple's brand is usability and a lack of rough edges. The downside of that is that building a tool like this up to their standards would be prohibitively costly.
I think the main problem would actually be the added storage consumption of hauling around old iOS system images, as would probably be necessary to keep the mainline version of iOS unencumbered from the constraints placed by backwards compatibility — it’d probably just be virtualizing old versions of iOS similar to how Mac OS 9 was virtualized in the early days of OS X.