The evidence to how absurd your claim is is right in front of you: Google's implementation of Protobuf uses std::map for dictionaries, and these dictionaries are exposed to Python. But, following your argument this... shouldn't be possible?
To better understand the difference: Python dictionary stores references to Python objects, but it doesn't have to. It could, for example, take Python strings and use C character arrays for storage, and then upon querying the dictionary convert them back to Python str objects. Similarly with integers for example etc.
Why is this not done -- I don't know. Knowing how many other things are done in Python, I'd suspect that this isn't done because nobody bothered to do it. It also feels too hard and to unrewarding to patch a single class of objects, even as popular as dictionaries. If you go for this kind of optimizations, you want it to be systematically and uniformly applied to all the code... and that's, I guess, how Cython came to be, for example.
