C libraries have advertised "header-only" for a long time, it's because there is no package manager/dependency management so you're literally copying all your dependencies into your project.
This is also why everyone implements their own (buggy) linked-list implementations, etc.
And header-only is more efficient to include and build with than header+source.
I never copied my dependencies into my C project, nor does it usually take more than a couple of seconds to add one.
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.