The problem with that is that aerodynamic drag grows with the third power of speed, so going faster requires much more energy, even after accounting for slipstream benefits that a self-driving mechanism could exploit.
For some numbers, see https://www.quora.com/How-long-can-you-drive-a-Tesla-S-P90D-... . The top comment is about racing, but the one below it lists 300Wh/km at 160km/h and 380Wh/km at 180km/h. That's a quarter more energy per km (!) for going 20km/h faster.
If the energy is free b/c it comes from the solar panels on the roof of my garage, why do I care?
Because it severely limits your range.
I live 8 miles from work. "Limiting my range" from 200 miles to 150 miles between daily charge ups is irrelevant.
Not to mention that battery tech will hopefully improve once everybody relies on battery powered cars, and some currently infeasible tech becomes cost-effective due to large scale deployments.
Which would you prefer? Going at 100 km/h with autopilot on today's highways, sharing the road with people without autopilot, or going at 250 km/h with autopilot on a highway where everyone else is also on autopilot?
We will have to follow a likely tortuous path to get from now to then, but I am totally convinced we will get there.