A vehicle coming from a side road has a lot of movement within your field of vision. When driving on a highway we're well used to the fact and expectation that stuff isn't stopped. When the vehicle ahead emergency brakes or stops you have the growth of size of the target to gauge by. That's not much movement, until the last moment when it's far too late to evade. Some stationary debris is much easier - it's not meant to be there at all - you transition to evasion right from the get go.
For a human in a car, the dangerous highway case is the stationary vehicle in the fast lane, maybe one or two vehicles ahead. When flying it's the other aircraft that's on a collision course. Neither will move within your field of vision. Neither is easy to detect as problem, until it's very late.
FWIW 4 year old humans often run into things. :)