Because you can knock a door with your hand down as well and it would be more convenient?
In: Physics
It’s more ergonomic. Your hands are still in a fairly neutral position (consider your palms) compared to lowering your arm, where you need to rotate your hand out so your palm faces the door and you can knock on it. When you raise your arm, your upper arm is doing most of the work, when you lower your arm, the smaller muscles in your forearm do the coordinating, again in a rotated position. Gravity also helps you knock on a door when your hand is up, vs when it’s down. You can knock on a door without rotating your hand, but that’s actually more awkward because now the outer muscles of your forearm have to pull your hand up, so to speak. Sometimes people knock like that, but only with a finger or two and usually just to get someones attention, not announce to an entire house that you’ve arrived since it’s not as loud as a “traditional” hand-up knock.
Also, historically, doorknockers are on the upper door near face level.
Source: I’m an industrial designer.
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