The problem lies in making things work with optimizations. For example, how do you schedule instructions and allocate registers optimally when you don't know th

The problem lies in making things work with optimizations. For example, how do you schedule instructions and allocate registers optimally when you don't know the exact sequence of instructions that is executed? Not gettimg that right costs you a bunch of performance. Then there are things like variants of the code that might be vectorized under certain conditions. You would need to find them in the compiler and be able to emit the proper alternative code templates and a way for the runtime to pick them up when appropriate. A full JIT could do all that at runtime, but that is also expensive to run and likely too big and buggy to exist in a stable and safe operating system kernel.