Why are bitwise shifts (>) used for cout and cin?

According to §8.3.1 of The Design and Evolution of C++:

The idea of providing an output operator rather than a named output function was suggested by Doug McIlroy by analogy with the I/O redirection operators in the UNIX shell (>, >>, |, etc.)

[…]

Several operators were considered for input and output operations:
the assignment operator was a candidate for both input and output, but it binds the wrong way. That is cout=a=b would be interpreted as cout=(a=b), and most people seemed to prefer the input operator to be different from the output operator.
The operators < and > were tried, but the meanings “less than” and “greater than” were so firmly implanted in people’s minds that the new I/O statements were for all practical purposes unreadable (this does not appear to be the case for << and >>). Apart from that, ‘<‘ is just above ‘,’ on most keyboards, and people were writing expressions like this:

cout < x , y, z;

It is not easy to give good error messages for this.