They do, it's just not a priority. The PEP I linked (as well as most of the work of its author, Victor Stinner, in the last years) are motivated by JITing.

There's also the "frame evaluation API" PEP [1], whose purpose is to allow pluggable evaluators in CPython without forking the entire interpreter, like Unladen Swallow had to.

[1]: https://www.python.org/dev/peps/pep-0523/