One piece you are missing is that the server side needs to be asynchronous. A well written mobile app will be doing things in the background as the UI can't be

One piece you are missing is that the server side needs to be asynchronous. A well written mobile app will be doing things in the background as the UI can't be coupled to network activity. There can be network glitches, temporary disconnects, abrupt closes of the app etc. If you have a crappy server API this pushes a lot more complexity onto the client, or it just makes for more ways to have different failures. A well designed server API works best with a well designed client, but being messy on either means more work on the other.