If you drop a library for internal usage only and you want to change the contract, and the tests, library, and api are all in the same repo, you just change them in a single PR and that's done. This works because the single commit hash contains all information. It requires people put their code in the monorepo and hookup their tests, but assuming they do, you can build a reasonable degree of confidence on a green build.
Once you separate the library different repo, and let people consume that as something in their own repos, you need to do the versioning dance as you have no idea if their still is still working.
A lot of people go for the latter because it contextually allows them to ignore the rest of the stack, and there are some pros to doing this, but testing, deploying, versioning, etc, all become more difficult, and that's something struggle with.
Thus, unless you have a crap load of code / commits, monorepos arguably have more advantages than disadvantages.