It is not just a "wrapper for the UI code" there is a shitton of stuff, just for starters there are all the sensors (gyroscope, gps, camera), permission handling, packaging the app, interaction between apps, background services and a lot more stuff that differs from one platform to the next.
Yes you can share much of your application logic between each platform using C/C++, but:
1) You need to write it in C/C++
2) If your app mostly just talks to a server there is not much application logic in the mobile device, so most of your code will not be portable at all.
There is a reason mostly only games share significant codebases between each platform. Their application logic is already usually written in C++ and they don't often have to deal with the stuff I mentioned before.