The question why people are corrupt is something can't be easily answered. The gene for corruption is yet to be identified. But as we understand human nature it is accepted that corruption is a natural trait and being non-corrupt might suggest a defective gene. A fine analogy could be smoking. Though the individual benefits of smoking and corruption can not be correlated, their overall effect on the society is generally negative.
As this article suggests, non-smokers possess a defective gene which prohibits them from smoking or controls the habit of smokers. This could be true for non-corrupt people. I have drawn a chart that tries to classify corrupts and non-corrupts based on their intelligence and attitude towards work. This attitude is generally a function of motivation, drive and passion.
Where, NC -> Not Corrupt
LC -> Less Corrupt
C -> Corrupt
NO -> No Opportunity for being corrupt