My answer to your initial question, at least as it pertains to front lawns, is that in America we afford drive-by shooters a sporting chance.

As for backyards, I must point you to the high-ranking that barbecue has among our favorite and patriotic pastimes.

In all seriousness though, the unused real estate between dwelling structures gives a sense of autonomy and privacy and power ("look at the land I own!"). I would venture to guess that the reason we do not convert our lawns into gardens often enough is because we clasp onto that image of traditional real estate wealth, and a very practical reason: gardening takes time and skill and an attention span that we have been willing to forfeit.