...but they should be. Unfortunately, for the past century various organizations have been campaigning to keep children off streets in the name of "safety." Bike Delaware has posted some interesting images, like this one, from early efforts to keep kids from playing in the streets--because those streets belong to vehicles, not people, right?
Lest you think that the auto lobby was entirely to blame for this, check out this 1914 NY Times article about "Movies" of Street Car Accidents Shown in Brooklyn Schools . (Yes, the battle for street space started when "movie" was still a slang term that had to be put in quotes.)
Sadly, children seem to have learned the lesson. Want a sign of how important private vehicle travel is to our culture? According to one recent study, of the 25 words every normal toddler should know before age two, only one has to do with transportation: "car."