I came across an interesting article about engineering education in the US.  Evidently, 50% of US engineering students dropout:

The United States graduates about 70,000 engineers each year, and another 30,000 with advanced engineering degrees… On average about 50% of U.S. engineering students dropout.

Other majors have about 25% drop out rates. In both cases many students choose another major…

I’ve found that most people I ask cite their reason for leaving as the difficulty of the courses.