Because most languages are highly similar and Google + your IDE is powerful enough to make up shortcomings. E.g If you have solid background in dynamically typed) languages (e.g. python, J's), GC static typed (Java) and lower level (C/c++ and/or one flavor of ASM), you've basically seen all ways of thinking. E.g. picking up something like Go can be done in a matter of weeks. Languages with more novel concepts though (e.g. That's memory model) will be much harder to onboard to.

> Is every C++ programmer a graphics programmer? Can a C++ programmer become one overnight? Highly doubt it.

That's different. It's the domain jump (be good at graphics) that is hard, not switching languages.