Why isn’t there a HN app?

Why should there be?

Reddit (for example) would have a perfectly fine mobile web experience except that it harasses you to install a mobile app that is worse than the web site. (Ought to have a steaming turd emoji next to the "call to action")

The app experience is almost always worse than a web site unless an app can take advantage of some special features of the platform such as high-precision locations in the Apple ecosystem.

I made an alternative HN interface that works good on mobile and adds some functionality like dark mode, split pane view for comments, etc..

It's open sourced and you can use it from https://yahni.news

FWIW, I’ve really enjoyed using Octal on iOS for the last couple of years.

Because HN's mobile UI is already great. Unlike other websites, HN's mobile UI has all of the same features as the desktop UI.

I don't know what problem an app would solve in this case.

There are already some great android apps available so I guess no need for any official app. Check out Harmonic and Materialstic for example.

there are many unofficial implementations & PWA webapps