Just a thought

I had a thought the other day that I've never had in America. I was in a cab driving past a construction site when I pondered: "Why is that building only going to be fifteen stories tall?"
This struck me as odd considering the last place I lived didn't even have a fifteen story tall building in the city. Here, just fifteen stories seems like a waste of horizontal space. If you're going to use up a square block on the ground you might as well make use of as much vertical space above it, right?