Yeah, 1/16th isn't nothing. However, the point is you're mostly accelerating sideways, not up. If that were a significant part of the launch then we'd launch from mountains, but we almost always launch from the lowest points possible, at sea level.

Yes, the Earth is rotating, but most of your orbital velocity is added during the launch. The altitude is pretty minor. It's why very few rockets have been launched from planes, that can give a significant portion of the altitude with very little cost. Altitude is just a small enough portion of the launch that the extra complications don't give it an edge.