Because they're doing what you're doing but the electric motor, regenerative braking, better cd, and so on make it possible to get really high fuel economy. Combine that with pulse and glide these guys are getting 1000 miles on a 10 gallon tank. In order for us to pulse and glide in our regular cars, we have to shift to nuetral, or put in the clutch and kill the engine on each glide phase which isn't practical as you'd be killing the engine and restarting it every 60 seconds or so.