There's a really big difference between cars and other hazards. All the other hazards for a cyclist are them hitting something which is a lot safer than them being hit by a vehicle that weighs a ton or two.
Hitting things other than cars is what helmets are good for. If you are in a bike-car accident no helmet will protect you - you need a several ton cage around you if you want a chance. (motorcycles face the same issue). However there are a lot of things other than cars that you can get in a bike accident with, and most of those are things where helmets are helpful.
I'm solidly on team helmet but F=ma is dominated by several tons of SUV. It's really hard for almost any other situation to reach the same energy levels and on average that's what determines how severe an impact is.
That's true if and only if the SUV is moving at a high relative speed. Otherwise an SUV will not out-inertia the Earth or anything bolted to it, like a concrete paver, a brick wall, or a lamppost. And yes, if the car hits you at a high speed, you're no better off than a pedestrian and the helmet is unlikely to help.
But without a helmet, it really doesn't require much energy for a head strike to be a fatal or life-changing injury. If riders face a baseline risk from non-car collisions, there's a strong case to be made for mandatory helmets, even if it means the risk from car collisions increase.
one thing you're missing is that the ground always hits you from the same direction, and you always have a bit of warning. the dangerous car hits are being hit from the side which can happen without any warning.
That is true only if the SUV is moving quickly. It would still be true regardless of whether the SUV weighs a hundred tons or a hundred grams. I'm not missing it.
