I don't think it's just that, though I agree they are clearly correlated.

The reason I don't think it's a sufficient explanation is that there is a clear history of large, 3rd party libraries being created exactly to supplement poor standard libraries. C++ has Boost, Java has Apache Commons (though Java also has a pretty huge standard library), arguably we could even say C has Posix/Win32/Cocoa.

I believe there is some deeper cultural reason why certain language ecosystems coalesce large utility libraries, while others prefer myriad tiny dependencies.