Sandboxing Python used to be and still is hairy and very hard (it requires intricate knowledge of CPython, because you are going to inspect the AST for unwanted things and do some other relatively low-level stuff) and Python is also a pretty big dependency (~5-10 MB). I think many recent-ish additions to Python may make sandboxing easier (e.g. audit hooks, subinterpreters), but it's still hard and messy.