Text selection is such a great example precisely because it is incredibly useful to have in many unexpected situations (and a great many more that should be expected), but UI designers as a rule do not think about these situations!
It is so bad that one of the most impressive operating system features to be added in recent years is the ability to select and copy arbitrary text from app UIs, using either accessibility APIs or (more recently) straight-up OCR (because of course accessibility is another thing UI designers forget).
It's not like adding text selection in native apps is even hard; it's just not on the radar, and never has been. The number of old-school apps that added some form of "open log file" to either support instructions or as an actual function in the UI instead of making error messages selectable / copyable is depressing; I've seen programmers spend more time mocking end users for not knowing how to take proper screenshots than it would have taken to implement selectable UIs.
...and by historical accident, this problem is now solved in the vast majority of new applications. A small mercy!
It's true, what people think of as native on the web are merely incidental from its history, not some ironclad law of how to make interfaces.