Motorbike helmets would give better protection but are too heavy and hot for cycling.
It's important to keep the risk and dangers in perspective when thinking about any activity. You need to bear in mind that cycling on roads is by normal standards a safe activity and putting on a helmet for the possibility of a vehicle collision is looking to protect from a rare event. By far the most common bike accident is the one vehicle variety - just the bike and its rider - and the majority of those are at speeds where a helmet may well save a sore head.
Cyclists have a longer average lifespan than non-cyclists, some of us are only alive as a result of cycling, that's the fitness difference it makes.
The perception of risks in cycling is much higher than the reality. I don't expect you wear a helmet when out walking, and statistically that's at least as dangerous as on road cycling! I'm not decrying helmet use, just saying that it's a nice to have extra, not a necessity.
I haven't seen many car drivers and passengers wearing helmets either, and at the risk levels for road use they aren't all that much safer (time based) than cyclists either.
None of the statistical analysis helps much if you're the person killed in a motorway crash, or who gets hit while walking across a pedestrian crossing, but it's important when thinking coolly about the likelihood of being hurt
Sometimes this forum doesn't help, as we tend to highlight every cycling fatality, without doing the same for other road casualties. There are roughly 9 non-cycling deaths for each cycling one.