I'm a qualified car driver. But I wouldn't class myself as an expert on the composition of asphalt on tyre grip capabilities based on varying levels of remaining tread depth vs road surface wear etc. Nor on the relative effeciency of halogen headlights vs newer xenon beam types in varying levels of darkness / fog / rain.
Of course you wouldn't and if that was a requirement of being given a driving license then very few people would have one.
If you increased the level of expertise required to drive a car to the level that the risks of accidents was virtually zero then the world would grind to a halt as most people wouldn't have a driving license. So people trade off the risk of dying in a car crash with the freedom it allows.
What has happened in the case of the volcano is that the vast majority of the public have made the giant leap from a volcano erupting to that of BA9 flying through a huge plume of ash.
I think jumbodriver and pretty much the vast majority of pilots that I am aware of and come into contact with on a daily basis, have decided that the calculated risk is so small that it's non-existant. But then that applies to pretty much anything anybody does nowadays, including getting onboard an aircraft even before the volcano erupted.