We sense approach or departure by the rate of angular growth or shrinkage of an object. This was originally couched as angular acceleration....perhaps more true at night when one might be able to only see two taillights while overtaking a slow, or stopped, object. Headlamps poses a more complex issue with being unable to look at them directly if too bright (hence the inherent lack of wisdom for high beams on a bike 24/7/365).
If the angle starts small, when it becomes slightly larger is probably more difficult to perceive. If the angle is larger to begin with, I think we can all agree that growth is easier to notice.
Another factor is the squish likelihood. How often do people fail to notice the 60,000 lb tractor-trailer that will give another vehicle a really bad day. Inherently knowing that you will get seriously messed up tends to elicit behaviors that help with continuing your genetic lines......for most.
Then again, some people are just stupid and not only pull out in front of motorcycles but, trucks as well.