In terms of organ donation, that is because an astonishing amount of people don't wear a helmet when on a motorcycle or wear one of those fashionable open-face helmets.
Many people are also unaware you can get airbags for motorcyclists (https://www.revzilla.com/motorcycle/alpinestars-tech-air-5-s...). This is obviously a thing more for first world riders and they are expensive but are very effective at preventing any injury you can't walk away from (assuming you also wear a proper helmet like I mentioned).
Cars have materials designed to deform in predictable ways to absorb shock and smooth the deceleration process in case of an impact.
Motorcycles have absolutely nothing of this sort. Body airbags can help a lot, but are still far from the safety that a car offers.
Personal opinion: motorcycles, as a means of urban transportation not sports or leisure, are for people who don't have an option and don't value their lives highly enough.
> "We were unable to identify any randomised controlled trials of parachute intervention"