C libraries are usually easier to integrate even into C++ projects because there's no "C++ subset incompatibility" (e.g. use of exceptions, RTTI, banned stdlib features etc... - since most of those things don't exist in C in the first place), and "flat" C-APIs are also usually much saner (some C++ library APIs have designed themselves into class-hierarchy- and/or template-hell).

There are easy to integrate C++ libraries like Dear ImGui but they are in the minority, and (for instance) Dear ImGui uses a very restricted set of C++ features (it's essentially "C with namespaces and overloading", so it's not very far removed from a C library).