Motorcyclists are hard to see when you’re on the road. Most drivers don’t watch out for motorcyclists. And most motorcyclists don’t seem to drive like they care if other drivers see them or not.
It’s easy for a motorcyclist to unintentionally hide in one of your many blind spots. I actually don’t have a side view mirror on the passenger side of my car. If I want to see what’s behind me on that side then the only thing I can do is turn and glance back for half a second. I really can’t spend more time than that looking back because I’m risking hitting something in front of me when I do. Half a second isn’t a lot of time to survey everything that’s going on behind me. If a motorcyclist is in just the wrong spot when I glance back then I won’t know he’s there. If I try to change lanes then I could easily hit him.
A lot of people check their blind spots as they’re changing lanes or turning. That’s a bad practice whether there’s motorcyclists around you or not. Combined with motorcyclists, you’re putting yourself and the other person at a great degree of risk. Both passenger car drivers and motorcyclists have to be diligent about watching for each other to make sure accidents are avoided.