I mean a pianotuneR (as in a guy that tunes your piano) is pretty expensive.
These apps seem to be marketed as tools for professional piano tuners. And looking just at the screenshots it looks like it has a lot of tools and features outside of just showing the correct pitch.
If tuning pianos is your profession, paying 999$ once and writing it off as a business expense isn't that far fetched.
Well, you can't just use any tuner app for tuning a piano, basically because intervals (multiple simultaneous notes) on pianos are always slightly out of tune and we just try to approximate them as well as possible (via a tuning system called "12-tone equal temperament").
If you'd tune each individual note to be perfectly in tune according to a normal tuner, then intervals would sound horrible (unless you'd only ever play in C Major, I believe).
But yeah, it's definitely still a matter of the user base being niche and the users making money when using your app.
So, the most expensive iOS apps are complicated applications aimed at professionals, while the most expensive Android apps are all junk apps aimed at rich people.