Why the convention prefer to use std::cout rather than cout [duplicate]

This question already has an answer here:

• Why is “using namespace std” considered bad practice? 34 answers

I understand that cout exist in the std namespace and without writing using namespace std; we cannot use cout, instead we have to use std::cout to include only cout from the standard namespace.

But my question is why most of the programmers I see they prefer to use std::cout why do they not just write using namespace std; what is in the std they do not want to include. They always write like this:

std::cout

std::cin

std::string

Won't writing using namespace std; be easier and faster ?