There's more gunshots and stabbings than people injuring their head on bicycles.
You can buy a suit of chainmail for the same price as a modern bicycle.
Its minimal cost.
In most countries, the risk of shootings or stabbings are orders of magnitude lower than having a fall or accident on a bike on the road with other types of vehicles.
In certain parts of the US, though? Yeah, wearing body armor would probably be a good idea. I think people just generally try to avoid going to those places as their strategy of risk avoidance, which negates the need for any body armor.
to be absolutely safe you should wear both full body chainmail and a bicycle helmet.
For me the main cost is having to carry around a bulky helmet when I'm off the bike and ugly indentations on my hair and forehead.
What is the COST of wearing a helmet? $60 purchase and 5 seconds every time you ride. Essentially nothing.
Compare to, e.g. TSA, which has a cost of billions of dollars and millions of person-hours. Unlike helmets, which are known to actually prevent some injuries, TSA is also effectively useless at its intended goal.