Riding bicycles on freeways - or even low-powered scooters like Vespas - would be INSANE. Traffic moves too fast and too many slow lane drivers are not paying attention, or lose control of their cars. CHP Officers and Cal Trans workers lose their lives on freeways all the time.
Riding your bicycle on a country road is a big risk too, but traffic is lighter and the speed limits are lower. If a car hits you on a rural highway, you at least have a decent chance of survival.