Life’s mysteries. 
Most people I know profess themselves afraid/unable to parallel park.
(I am not one of them.)
Through this very unscientific poll, I am going to assume that 80% of all people driving have the same fear/inability.
So, why do these same people, who are afraid/unable to move their car sideways into a spot at less than 5 mph, have no problem whipping their car into an area (much smaller than a parking space) between two other cars that are travelling at 70 miles an hour on the highway?
(What an awkward paragraph.)