>Because it's [stereotyping is] easier than critical thinking?
In a way, yes.
In fact stereotyping is like writing a script rather than repeating the same process over and over. You may need to amend the script for a given situation of course, but it serves as a useful first approximation of a solution.
It is critical thinking that leads one to effective use of stereotypes: it's not always raining when it's cloudy outside but it's a useful indicator.
http://en.wikipedia.org/wiki/Heuristic