Then you lose the capability to atomically make a commit that crosses repoes. I'm not sure if there is any forge that allows that, except Gerrit might with its topics feature (I've not gotten the opportunity to try that).
Let's take for example a service 'foobar' that depends on in-house library 'libfoo'. And now you need to add a feature to foobar that needs some changes to libfoo at same time (and for extra fun let's say those changes will break some other users of libfoo). Of course during development you want to run pipelines for both libfoo and foobar.
In such 'super module' system it gets pretty annoying to push changes for testing in CI when every change to either libfoo or foobar needs to be followed by a commit to the super repo.
In a monorepo that's just another Tuesday.
Again, tooling issue. CI can easily pull required changeset across multiple repos. We are in a subthread under "monorepo plus millions of dollars of custom tool engineering" vs "stock polyrepo"