Why are stairs not replaced by ramps?

If ramps allow accessibility to a larger number of people as opposed to stairs, then why are stairs more common than ramps in public buildings (or buildings in general)?

Comparing a reasonable angle of incline for a ramp to a set of stairs, the stairs have a smaller footprint to get the the same height. Is this perhaps the reason?

Perhaps stairs are a more comfortable method of moving up and down levels if you are capable of using the stairs. That would help explain why ramp and stair hybrids exist (such as the one pictured below).