I think there are two issues. In C++, a header only library makes sense due to templates, which don't produce object code before specialization. In C, I have seen the same with entirely macro-based "libraries" that are really simulating templates in order to make abstract data structures.

The second case though, my theory is that there is a new generation of programmers who don't understand the traditional compile and link phases because they are used to other languages. It doesn't fit their expectation of how it should work so they bend the tool to their expectation instead of figuring out the old way. The very fact that people on here are saying a programming language should have a "standard package manager" demonstrates the cultural divide: this sounds a little nutty to an old time C person.