Windows is no excuse, because many of "single-header C library" sinners are UNIX only folks, targeting only UNIX platforms.

For me it only reveals lack of willingness to learn how to use their tools.

I learned how to use C and C++ build systems at the age of 12 years old, when most of the information was on libraries, or whatever folks down at computer club could explain.

In a computing landscape much more fragmented than it is today?

Do you think combining Windows and UNIX is hard? Try on the glory days of 16 bit home computers.

Here we are in the age of information, and some people couldn't be less bothered.

Nope, most people using it know how to link and compile libraries. :)