What? What makes you say that? SpaceX literally shot Elon Musk's car into space to do nothing but float towards Mars. The entire point is prove that their rockets can shoot something that weighs a metric ton out to Mars safely. They don't want to send something lighter that's only designed for travel to and on Mars. They want to send something from Earth to Mars that proves they can send out the materials and supplies to colonize the planet quickly.
All your points about the benefits of an electric vehicle apply just as well to something lighter that is specifically and only designed for travel to and on Mars.