Just because things change in tandem, that does not mean that they're all the same thing. When I add a new function to my backend service, all frontends that consume its API also need to be adjusted. But that doesn't mean that the backend service, its command-line clients and its web GUI client should live in the same repo.
It's probably a matter of taste - but I think they should be in the same repo. I like tying test failures/regressions to a specific commit for documentation and admin purposes. Having a test fail or regression due to an 'unrelated' commit in another repo sounds like a nightmare waiting to happen when you try investigating.
I the difference of opinion is between developers who work on self-hosted "evergreen" products where the latest version is deployed, and others who work with multiple release branches with fixes/features constantly being cherry-picked.
Why? You are just creating more work for yourself by keeping the components in different repos. Now you need to create N commits when updating something. If your future self wants to investigate how the software has evolved there are N times as many commits to analyze.
