Obviously, I am not saying making a JIT for Python is impossible: both PyPy and GraalPy prove otherwise. We are talking about full ecosystem support, not just the base language and stdlib.
Can you just replace CPython with PyPy or GraalPy in every project? And why not? :)
Because the Python community writes libraries in C, C++, Fortran and Rust, to make up for the CPython slowness, using CPython C API, calls them Python libraries, and they naturally only work in CPython.
PyPy and GraalPy then never get to use those pseudo "Python" libraries, because their API for C based extensions differs enough that you cannot take a random shared library and load them into either.
They came up with a common C API proposal, but has hardly taken that much adoption.
Indeed, mixing all the language features and anti-features with non-native Python libraries is a big reason too — both because it mitigates many of the performance issues a JIT would solve, but mostly because it complicates or makes the implementation unfeasible altogether.