To be clear, this is not meant as a critique on C, on the contrary. Put simply, it is just that making computers do stuff is hard. This is especially true if you want it to do it in a very particular way such as in high-performance code. If you cannot already leverage the work of other experts, you need to know about modern optimizing compilers, language runtimes, modern processor architectures, OS/system, ...)

To continue your Numpy example, that framework entombs the know-how and man-years of work of specialist in a certain area. If you need to do simple dense algebra, you will a hard time beating Numpy/Julia/Eigen/MKL etc. If your problem goes slightly off-track (e.g. sparse problems) where you cannot easily leverage the hard work and expertise of others, you better start rolling up your sleeves and open that main.c