Speed is a factor. A motorcycle can easily travel 60km/h - 120km/h. A bicycle is likely to be travel at 20km/h - 40km/h. 40km/h is pretty damn fast on a bike. Maybe the enthusiast guys riding the beach in the morning get up to 60km/h but that's a pretty small percentage riders.
An impact above 60kph is beyond the design of even a motorcycle helmet. They are primarily meant to protect the head falling onto the road surface and then sliding, an impact that it similar no matter the speed. Even when standing still, a head freefalling from 2 meters (ie bicyclist/motorcyclist tipping over) strikes the ground at 22.5 kph. That single impact is the US/DOT standard for bicycle helms. They don't accommodate sliding or repeated impacts.
Not true. Look at MotoGP crashes. They’ll hit the deck at 150mph, slide off into gravel, slide to a stop, and pop right up with nothing more than a few scuffs on their leathers.
