Obviously if they paid less they would get fewer engineers, but there are plenty who would take peanuts to work on problems at that scale. The actual reason they're paying that much because they have no limit on the number of 1% engineers they can put to work, and they want as many as they can get.
FAANG companies aren't paying that much because they get so much more value out of employees: they are paying that much more because that's how much extra they have to pay to convince people to take the jobs they are offering.