Well. The most common problem is that people are running old stuff on their computers that lack the necessary security patches installed. Some people have it like this cause they are lazy, but most haven’t got their systems updated simply cause they don’t know that their computers are being used by others.
Secondly, the direct profits from stealing the information on these computers is quite huge. The estimates were quite rough but since this is an automated process the people behind it are sure to make their buck just from the theft.