Well for one it's a ridiculously oversaturated major:
http://www.psychology.heacademy.ac.uk/docs/pdf/p20050412_EnhancingEmployability.pdf
"less than 20% of psychology graduates continue on to professional training, with the majority seeking employment in other or related fields"
And a lot of the time you have to complete graduate studies in order for your degree to mean shit:
http://money.cnn.com/2000/12/08/career/q_degreepsychology/
"With more than 40 percent of undergraduates in the field eventually going on to law school, business school or some other professional program, the social sciences major ranks among the highest in post-graduate academic attainment."
Plus it's just pretty 'weak-minded' as far as the sciences go. When you take chemistry or physics there's this very complex and elegant set of theories, formulas and mathematical equations to explain what goes on in the world. Psychology, on the other hand, often bases its knowledge on things like... questionnaires. Hmm...