[throw14082020]

There's a lot that goes on in a mobile app, please read https://developer.android.com/docs/ and https://developer.apple.com/documentation

It certainly is different to backend development, but not everything is a "JSON viewer", even if you exclude camera filters and games. Only the truly awful apps are "JSON viewers". There is a lot of complexity, and I've heard arguments from people that 'they don't want to be a front-end dev because that is just visual stuff': they're talking about pre-javascript days or talking about something they've never done to a good level.

Using a backend (adding network cost)/ sharing code should be the last on your mind for mobile apps. I've had once, where the logic for feature would take less than a day to implement (which I subsequently did), but inexperienced non-mobile developers decided it would be a good idea (and said exactly what you said) to put everything in the backend to share code between iOS and Android. The interface cost (network code) cost more time in development, and made a much worse user experience. Not to mention, I had to use the code written by the inexperienced person, which had bugs.

You might not understand mobile, but there's a lot more that goes on in a mobile app.