Actually, some aircraft have windshield wipers, some do not. Aircraft designed to fly extremely fast, or alternatively, very cheaply, do not. Rain, snow, etc. is simply sheeted off of the windshield. In aircraft that are called on to fly in icing conditions (all commercial aircraft, many others) have heated windshields to melt off ice. Wipers and the mechanism that operates them are expensive, and prone to breaking at high speeds. Therefore they must be very sturdy, ergo, very expensive. Commercial jets that have wipers use them only on the ground where the droplets of water are not blown off