Anecdotal and impromptu statistics have lead to the proposal of the following 3 reasons why so many squirrels die by being run over by cars.
This theory proposes that squirrels evolved brains used to dealing with objects moving a relatively slow speeds. When they encounter a car it is moving too fast for them to comprehend its velocity. Thus a squirrel can not consider that it will arrive before it has completed crossing the street. Alternately a squirrel already crossing a street when the car approaches but take so long to interpret what it is seeing that it does not move out of the way.
This theory proposes that squirrels, which normally only need to see far enough to jump to a near branch or find a nut in the same tree, can not see far enough to avoid a car. An animal that can only see 2 meters away will be unable to cross a street where 5 meters of vision is necessary to avoid an on coming car.
This theory proposes that squirrels are frequently hit by cars because the squirrels are suicidal. They want to be hit by cars. And given that a squirrel's life consists of living out doors, subsisting on nuts and jumping from tree to tree, can you blame them?