Individual libraries/dependencies get worked on by themselves, with an API that other applications use. Then the other apps just bump a version number and get newer code.
That is, the reason an API changes is because a use site has need of a change. So, at a minimum, you need to make that change and test it against that site in a somewhat atomic commit.
Then, if the change has any affect on other uses, you need a good way to test that change on them at the same time. Otherwise, they will resist pulling this change until it is fixed.
Add in more than a handful of such use sites, and suddenly things are just unmanageable in this "manageable" situation.
Not that this is "easy" in a central repo. But at least with the source dependency, you can get a compiler flag at every place an API change breaks something.
And, true, you can do this with multiple repos, too. But every attempt I have seen to do that just uses a frighteningly complicated tool to "recreate" what looks like a single source tree out of many separate ones. (jhbuild, and friends)
So, if there is a good tool for doing that, I'd certainly love to hear about it.