The point I am trying to make: most of things flying seem more dangerous (higher speed, higher reach, different fuel) than cars.
And yet somehow flying is safer than driving! As a sibling comment mentions, a lot of that has to do with all of the safety controls in place.
My point is that driving at modern speeds in modern vehicles is actually pretty dangerous, even if not as dangerous as aviation, and yet we as a society exercise very little oversight over the activity.
There are good reasons to not have "ADS-B for cars", mostly related to privacy. Unlike flying, many people heavily rely on cars to carry out their day-to-day activities. But I think it's at least worth thinking about.
Hanging around FBOs/flight lines/bars with pilots that also ride, it's come down to this:
After countless attempts and arguments while trying to normalize accident/fatal accident rates by converting hours flown/miles ridden, statistics do support that GA does seem to be a bit more dangerous than riding.
That having been said, my opinion is that most of the GA risks are in the hands of the pilot, while most of the risks of riding are external to the rider. Most of the worst motorcycle accidents I've seen or heard of involved the rider getting schwacked by a car or truck, while most of the worst GA accidents involved the pilot schwacking him or herself with poor judgement and/or skills that are below MIF. I put a lot of effort into minimizing pilot-induced risk when I fly, so scientifically speaking and all, I -feel- safer when flying than riding, and probably am.
I believe that plenty of people would object to that level of tracking citing privacy reasons.
(I'm not against ADS-B myself.)
But I do think there's a difference in risk. Aviation has a pretty dang good record, and I think that the tradition of transparency in comms and telemetry is a huge part of that.
While this is true about commercial airliners it is not true for general aviation. The safety of general aviation is roughly comparable to motorbikes.