What really happens if you open up the code base: Mediocre people who want something for their resume flock to it, work on unimportant issues, add bloat, CoCs and generally make your life hell.
If you think mentorship and technical review committees can solve anything: They cannot. People who are warming the chairs in those committees never do any actual work on the issue trackers and try to bully the real workers.
You presumably haven't encountered this issue with your own open-source projects, but that doesn't mean the problem doesn't exist.
Nobody is denying anyone. The code is there to read as an example. If an aspiring developer hasn't got the initiative to develop at least a basic level of competence on their own, that's their problem. What happened to self-reliance?