That is simply not true. Lots of build systems assume certain things in certain places and output to be something (what if you want a .so but the build only spits out .a etc.). In any case you may have to resort to shell scrips or launching shell scrips from your own build system. In short: it sucks.

And there is also the case when the dependency has dependencies itself. Adding a dependency is one of the biggest PITAs in any C/C++ project. For these reasons:

1. No standardized build system.

2. No standardized use/configuration of common build systems (everyone uses CMake/SCons/whatever slightly differently)

3. Dependencies may have other dependencies, which use yet another build system (see 1. & 2.)

This is the reason why 'single header library' is a 'feature' C/C++ devs look out for when deciding on taking on a dependency.

It makes adding such 'libraries' as deps a no brainer as far as the above is concerned. I.e. your average Rust crate is to your Rust project as is a single header C/C++ to your C/C++ project. Every other kind of dependency is potential pain.