There's a number of extremely shitty vendor toolchain/IDE combos out there that make adding and managing dependencies unnecessarily painful. Things like only allowing one project to be open at a time, or compiler flags needing to be manually copied to each target.
Now that I'm thinking about it, CMake also isn't particularly good at this the way most people use it.
They are certainly bad vendor toolchain, but I want to push back against the idea that this is a general C problem. But even for the worst toolchains I have seen, dropping in a pair of .c/.h would not have been difficult. So it is still difficult to see how a header-only library makes a lot of sense.
One of the worst I've experienced had a bug where adding too many files would cause intermittent errors. The people affected resorted to header-izing things. Was an off-by-one in how it was constructing arguments to subshells, causing characters to occasionally drop.
But, more commonly I've seen that it's just easier to not need to add C files at all. Add a single include path and you can avoid the annoyances of vendoring dependencies, tracking upstream updates, handling separate linkage, object files, output paths, ABIs, and all the rest. Something like Cargo does all of this for you, which is why people prefer it to calling rustc directly.
Now that I'm thinking about it, CMake also isn't particularly good at this the way most people use it.