Motorcycle accidents happen often for a few reasons. The first is that drivers aren't always watching out for motorcyclists and may make mistakes that result in a motorcycle-vehicle crash. The second is that due to physics and the way the brain translates information, drivers may believe motorcyclists are traveling slower than they really are. The drivers don't give themselves enough time, which the results in a crash.
One thing you may question about motorcycles is why motorcyclists don't wear a seat belt. Strapping in, you'd think, would be safer than being thrown from the vehicle in a collision, right?
Wrong. In fact, the safest thing for motorcyclists is not to strap into the motorcycle. The reason is that being on the motorcycle during and following a collision is the worst place to be. It's not stable, which means the rider would likely be crushed by its weight and pinned. If you are thrown from a motorcycle instead, the impact with the ground does cause injury, but it only decelerates you slightly. The hope is that you'll slow over time, reducing your risk of severe injury.
If you do use some kind of harness, the likelihood is that you'll end up pinned under the motorcycle in some way. This can lead to crushing injuries, lacerations and even burns. When you consider the possibilities, being thrown is one of the least dangerous.
Crashes happen, but understanding the physics and why they are as they are can help you learn to handle a collision as and after it occurs. The right training could help you reduce your risk of injury.