Many, many reasons.
For one - imagine multiple Reginald Denny incidents in every town and city across the country where the zombies comprise a significant part of the population.
That scenario is not too far fetched once the country goes bankrupt and the handouts have to be cut back.
It still seems a bit far-fetched to me. Humans wouldn’t stand idly by while zombies caused their children to starve or be without medicines. To the contrary, humans would declare open season on zombies and they’d dust off the zombies in short order. There are millions of privately held weapons in the country. I have a little more faith in the strong right arm of your average American than most, perhaps. But given our history I doubt that it’s misplaced.
