Road cycling is stupid dangerous even without cars. People descending at 70km/h wearing nothing but Lycra and a bit of polystyrol on their head...
GCN has over 3 million subscribers on youtube, they post highly informative road bike videos, often with people who competed professionally at the highest levels. Still, they don't wear any protective gear rather than the helmet. For someone who just starts biking and wants more info about the sport, will look at these videos and don't even know about the existence of such a gear or the need for it.
That's just one example, but literally all articles I've read in the past 10 years or so are picturing slim, fit, cyclists wearing just lycra.
It's still a big question mark for me.
That’s the reason!
For downhill mountain bikers, full protective gear is standard, but I guess they don't ride for hours, and they crash more regularly.