Same as any other business. If banks want to make loans, they have to have deposits that people don't want to touch. If you want to make money on the stock market, someone has to make the opposite bet against you. If you want to make money selling airline seats, you have to assume that you'll sell every one.

Assuming other people will do something (or are wrong about something) is the basis of a lot of businesses. Nearly all of them, in fact.