you can do that with a browser too. with service workers, it can also run without an internet connection and/or indefinitely cache the ui part so that it's also just a json api. most websites already work in a very similar way, and even if it's not intentionally set up this way, your browser will do its best to make it like this to keep your user experience snappy.

your browser just also protects you from certain level of system access that shouldn't be granted to any random website you visit, and that's what these apps want.