Mechanical solutions often weigh more, as well. We're supposed to be designing for better gas mileage, aren't we?

Good gas mileage is one of the many design goals. Another important one is reliability.

and theoretically a computer-driven windshield wiper would be able to respond to weather conditions better than a purely mechanical one, improving safety (perhaps marginally, but that's ok by me). I really appreciate my auto-rain-sensing windshield wipers.