However, it is extremely common because it works on any platform. It's great at filling in for platforms where you don't get forced to use the first party one (like Xbox).
There's Unity and Unreal games on the steamdb listing, but there's also several other proprietary engines, both big (Source, RED) and small (all sorts of small indie games)
does this invalidate the idea? i get the core idea, there _are_ some libraries that nobody has ever heard of but for example used in just a few places, one of them being lets say jquery and that creates a misleading example of being some super popular library.
curl is not one of these examples, it is very, very, widespread.