The main reason IME for a header-only C++ library is that when it is template-heavy (as a lot of C++ code is these days), a lot of it ends up in headers anyway,

The main reason IME for a header-only C++ library is that when it is template-heavy (as a lot of C++ code is these days), a lot of it ends up in headers anyway, so it's not a big leap to header-only. I've never seen a widely used header-only C Lucas library, and while it wouldn't surprise me, it seems harder to justify.