Anyone who hasn't ever driven above 100mph, or driven a fast car/done track driving probs doesn't really know what they're on about. 120 in a fast BMW is likely safer than 90mph in an old banger. Obv not the smartest move but not the drama that some are making it out to be.
Or perhaps some of us or our loved ones have been victims of drivers who caused crashes at high speeds?
120 in any car in public isn't safe (completely different ball game to track driving so no idea why you're using that as any comparison) full stop, because no matter how excellent a driver the speeder is, there are too many variables outside of their control that mean at that speed they could not maintain control of the car.
Other drivers being shit, an animal running into the road unexpectedly, a pedestrian running into the road without looking, debris in the road that isn't very visible... all things that are much, much more dangerous going at 120mph.
Anything that could cause a crash at any speed is far likelier to cause a fatal crash at that speed.
This isn't a 'they don't like it because they don't understand it' thing, it's a 'yes we do understand it which is why we know it's selfish and idiotic... let alone with a child in the car' thing.