People scam to make money pure and simple, they feel that the need for money is more important than their morals and so they deceive others out of their money.
In my opinion, the only reason due to which people scam is none other than this that they find it quite an easy thing to grasp the money of others by using different kind of tricks. Their only motive is to earn a good amount of money which can help them in living a better life. They keep on working on different kind of schemes which help them in taking the money out of others pocket.