Why front lawns?

In a recent post on Planetizen's group blog, Todd Litman discusses the pros and cons (mostly cons) of lawns.  

It seems to me that there is a difference between front yards, back yards, and parks.  Back yards are useful as a play space for children (though in an age when children tend to prefer video games and similar forms of indoor entertainment, this benefit is not as important as it once was).  Unlike front lawns, back yards are insulated from traffic and obnoxious strangers of various types.  For the same reason, a back yard may be more useful for some families some of the time than a park.  

By contrast, front lawns are not particularly private, and in my experience they are rarely used for any legitimate purpose whatsoever, either by children or adults. They don't neccessarily have positive ecological consequences, since they are chock full of chemicals.  They take away space that could be used either for housing or for more expansive back yards.  Why do they exist?

Basically because most zoning ordinances require single-family houses to be 25 feet or so from the street, and those 25 feet are usually gobbled up by lawns because that is the least aesthetically offensive thing you can put on those 25 feet.  Why not just abolish setback requirements so people can build up to the street?