Why are Facebook lead form ads cheaper than sending people to my own website?

With a Facebook lead form ad the person never leaves Facebook. They click the call to action (such as Learn more or Sign up) and answer your questions inside Facebook.

This is one of the cheapest ways to get leads. Facebook makes money by keeping people on Facebook as long as possible, so it charges a premium to send someone off Facebook to another website such as your own, and rewards you with a lower cost per lead when the person stays on Facebook.