Then why is it so common for people to try and select a subset of the language. Arguing about what people should do is pointless, you have to look at what they do do.
So when you go to another project that made different decisions, what you should do is understand the decisions. It's quite easy. And that's why it's what people do do, and it's why people are capable of contributing to projects other than their first.
If you program in C++, it's the same skill you used to learn one or more of {CMake, Makefiles, scons, bash, python} except it's easier because you already understand the programming language's model.