why motorcycles don't have windshield wipers

The snarkier of my readership will certainly respond to my title with "because they don't have windshields" (although some do) or "because only idiots ride motorcycles in the rain". I rode my motorcycle in the rain today; mine does not have a windshield. The analogue of a windshield on my bike is the faceshield on my helmet, upon which rain does accumulate.

In the past I have tried two techniques to combat the rain on my faceshield. The first is to wipe off the rain with my glove. This is the worst thing to do. It both makes your glove wet† and leaves a more opaque smear of rain then you began with. The second is to lift the visor. This works well at low speeds, where the lifted visor provides a front porch from which you can look without getting rain in your eyes. At high speeds this is a lousy solution (as anyone who has ever tried sitting on their porch in a moderately windy thunderstorm knows). Today I found the high-speed solution. Simply twist your head slightly to the right (or left). This effectively turns half of your visor from a windshield to a side window. Side windows don't have windshield wipers for a reason - rain blows right off. A brief twist of the helmet to each side quickly blows the rain of the visor and one is free to navigate once more. With a little practice this can be done while still keeping one's eyes forward. Brilliant! Although perhaps not as awesome as this...
^† At least as important keeping a clear visor is keeping the rest of oneself dry. For this I recommend REI elements rain pants and a waterproof softshell (mine is Mountain Hardware). Both work great. For shoes I use Vasque's Sundowners. The jury is still out good waterproof motorcycle gloves.