That's true, but git also doesn't have tags that apply to a subset of the repository tree. You can easily check out `project-b/v1.2.0` and build project-a from that tree. Of course, the answer to that is "don't do that", but you still have the weird situation that the source control implementation doesn't match the release workflow; your `git describe` example is but one of the issues you will face fighting the source control system -- the same applies to `git log` and `git diff`, which will also happily give you information from all other projects that you're not interested in.

For me, the scope of a tag should match the scope of the release. That means that a monorepo is only useful if the entire source tree is built and released at the same time. If you're using a monorepo but then do partial releases from a subtree, you're using the wrong solution: different repo's with a common core dependency would better match that workflow. The common core can either be built separately and imported as a library, or imported as a git submodule. But that's still miles ahead of any solution that muddles the developers' daily git operations.