This is due to the unique design of their wings.
At the leading edge of an owl's wing is a sharp serration that slows the flow of incoming air during wing beats, redirecting it to the side and thereby reducing noise levels. The trailing edge of the wings is also equipped with a 'muffler' — a fluffy fringe that eliminates turbulence created by changing airflows behind the wings.
Additionally, the soft down covering the owl's body effectively absorbs all sounds generated during flight. Therefore, it is nearly impossible to hear an owl, even if it flies so close that you could reach out and touch it.
