At this point, the biggest problem simply seems to be that the size of "Python Core" has outstripped the number of maintainers.
I helped shepherd a bug fix into Python that was less than a dozen lines, dead simple, completely obvious, sorely needed and still took 3 years and a summoning of Guido, himself, to get it shoved through. Because there was no designated maintainer for that section of code, people were absolutely terrified of touching the code even though it was completely obvious that the fix was backwards compatible. It finally hit the latest Python and a bunch of other projects immediately removed their workarounds for the bug.
If it was that difficult to get a super small, super obvious bugfix through, trying to get a "frozendict" into the language is going to be a Sisyphean task.
There's a lot of changes going on in CPython in the past few releases, so a frozendict wouldn't be a big one comparatively speaking.
The bigger problem is that there was already a PEP (https://peps.python.org/pep-0416/) about that, and it was rejected for wonderful reasons such as "multiple threads can agree by convention not to mutate a shared dict, there’s no great need for enforcement" and "there are existing idioms for avoiding mutable default values".