Why do you guys pursue a flat frequency response when you buy a subwoofer?

As we all know, most audiophiles spend a fortune for that flat frequency response displayed on the manufacturer's specs when you buy a subwoofer. Why do you do this? The minute you put that flat sub in your room and take some measurements, it is anything but flat (it's a rollercoaster with all kinds of peaks/nulls etc, EQ to the rescue).....So, why do you dudes continue to look for the flat line? What's going on in your mind when you're shopping around?