Part of the reason is that it's so fun to go fast with them.

For one, more strictly enforce technical fitness and some form of age requirements. An awful lot of people don't care much about the roadworthiness of their vehicle, and many don't care about technological advances like anti brake-lock systems in newer models as well.

The other major contributor to motorcycle (or bicycle) accidents is road conditions like potholes, dirt and especially oil contamination, bumpiness in roads... a car doesn't care much (unless it's one of those super-flat sports vehicles), but a cyclist can easily lose control.

On a scooter not, but on a Harley or Honda Gold Wing? People make trips across Europe on these beasts.