- The vast majority of code generation would have to be dynamic dispatches, which would not be too different from CPython's bytecode.
- Types are dynamic; the methods on a type can change at runtime due to monkey patching. As a result, the compiler must be able to "recompile" a type at runtime (and thus, you cannot ship optimized target files).
- There are multiple ways every single operation in Python might be called; for instance `a.b` either does a __dict__ lookup or a descriptor lookup, and you don't know which method is used unless you know the type (and if that type is monkeypatched, then the method that called might change).
A JIT compiler might be able to optimize some of these cases (observing what is the actual type used), but a JIT compiler can use the source file/be included in the CPython interpreter.