My understanding is that this is because they are looking at the thing they don't want to hit.
I've read advice for cyclists that if they are trying to avoid a pothole, dead racoon, whatever... they should look at where they want to go and not where they want to avoid.
Seems reasonable.

I've mentioned my experience of nearly getting hit by a car approaching me from behind. It was night, there was no other traffic, and there was no reason for the driver to not just go around me. I saw him in my mirror and could tell he wasn't pulling to the left. I was moving off of the road and onto the shoulder when he went by and knocked off my left pannier. He came back to check on me, and during our discussion, he mentioned that he saw my tail light. My guess is that he was staring at the tail light and just drove towards it. I think that has been the explanation for people driving into police cars that are stopped at the side of the road. People start looking at the car and drive towards it.

... oh... almost forgot. I was riding a recumbent on a back road on a quiet weekend morning. I was going up Santa Fe road, and a car was approaching from the other direction. He was looking at me and drifting across the road and headed towards me. I signaled for him to get back on his side of the road, and he sort of snapped out of his trance and moved back over. Was I imagining that he had a sheepish look on his face??

These examples lead me to think that bike lights should be bright enough for people to notice you, but not bright or weird enough to make people curious about them. You don't want people looking at the light more than a second or so.

Steve in Peoria
(disclaimer: this data is clearly anecdotal and should not be considered conclusive)