>There’s no reason why CI in a monorepo can’t create versioned code artifacts like Python packages, Java libraries or special jars, Docker containers, whatever. This is a very common workflow, e.g. combining a monorepo with in-house artifactory.

Correct, and this is necessary. But there's no reason for a to depend on b from the artifactory instead of a just depending on b at a source level, and building a and b simultaneously and linking them together. Now you have fully hermetic, reproducible builds and tests.

Why is not doing versioning so insanely bad that you can't list all the reasons (this would be a much more interesting discussion if you did).