Because choosing a sales job means that you have to face the pressure of performance and the changing market every day. It also means that you have to face all kinds of customers with different personalities, good and bad customers, and all kinds of competitors that you can't even think of.

This is not an easy job. But once you get familiar with it, you can get far more than choosing a stable non-sales job. For example, higher income, faster promotion, relatively fairer working atmosphere, and so on.