Because the flange was necessary to be able to get the bolt and nut started with a standard socket. The bolts go in from underneath up in to a deep recess. If you ever tried installing somthing like that without a shouldered bolt, then you know you have to stuff one or two (sometimes more depending on your socket) nuts in the socket to keep the bolt from going so far down that you can't get enough threads through the other side to get the nut started. They couldn't just use a longer bolt, or it would have interfered with the floor of the car after install.