Whether runtime generated or downloaded from the internet, you cannot run instructions directly on the processor without requesting executable virtual pages in memory from the OS. Executable pages are also a major security vulnerability: https://en.wikipedia.org/wiki/JIT_spraying

An App is not just code, it is an entity within the OS with permission system, metadata etc all attached to it.

A JIT compiler is not above the App’s sandbox permissions.

They look into the app as a human vs looking at the code. iDOS 2 is a great example. Banned for doing that exact thing.