Looking at my peers, who mostly have science and engineering backgrounds, there is a perception that we need to work in "our field". Not using our degrees makes the entire expense and time of college seem like a waste[1], even though the degree can still help get unrelated work.

On top of that, we treat searching for jobs in a very focused way: I'm looking for a job in "x". I'm sure there are lots of other jobs, but if I never think to search for them it's like they do not exist.

1: http://en.wikipedia.org/wiki/Sunk_costs#Loss_aversion_and_th...