Is there any reason C++ 11+ std::mutex should be declared as a global variable instead of passed into a std::thread as a function parameter?

I've seen most examples using std::mutex where the mutex is global. I was wondering is there any specific reason why this is done? I've had programs of my own where I don't do this, and simply pass the mutex in as a std::thread std::ref. Isn't it bad practice to have globals, what is the rational behind global std::mutexes in C++ if there is no language restricting reason to do this?