Flatlands are all about you and the bike. There's no terrain to lower your speed, no climbing, then descending, it's all about how fast you can pedal your butt. The only factor of nature is the wind. Ride on a calm day and the max speed on your computer is something you attained on your own, not with the help of a hill.
I live in an area that has the good fortune to have both hills and flats. If I head west or north I hit plenty of hills, some with names well known by all the local roadies. If I head east, I hit the flat lands near Clayton, NC. During the week, when I don't have time to drive east, I ride the hills to stay in shape. On the weekends when I'm doing a long ride, I head east, to ride the flatlands. I'll take the flats any day.