Others have touched on the probability thing.
The issue with motorcycles is that a some of it is under your control (driving safely, protective gear, bike maintenance) but there's a lot that isn't: potholes, other drivers, animals and so on.
Flying, almost everything is under the pilot's control. That includes most plane failures. Good preflight and maintenance takes care of most issues. The rest is taken care of by the flight planning – for example, engine failures. You should always have a place to put down the plane at any moment if you lose an engine - and general aviation aircraft land pretty slow.
Newer advancements have made it even safer (see also, whole frame parachutes).
That basically leaves freak accidents; they are a minority. Go spelunk the NTSB database, you'll find most accidents were preventable.
In a nutshell, you are probably going to find the risk is very skewed by complacent or otherwise irresponsible pilots.