Many open source projects are hard to build, especially C/C++ ones because these languages don't have a standarized package manager and a proper package ecosystem. C/C++ projects often use Makefiles which rely on bash commands. If you're on Windows, good luck. Sometimes you get lucky and there's a CMake file which can create a Visual Studio solution for you.

With languages like Rust, D or any webdev project it's much easier, because you can go "cargo build" and it will autodownload dependencies and build everything, no matter the OS.