Each ride, and rider is different.

Most of my longer day rides are relatively flat, but that still doesn't equate riding 15 miles to riding 150 miles. You may get some hills, but the rides are still, at least different. And, it takes a lot of general endurance to do a 150-PLUS mile ride.

Also, what you consider hilly may be killers for some riders, and pretty routine riding for other riders.

I've done a few semi-organized half century rides, and they certainly are not flat. The organized century rides I've done also seem to find good sized hills to go over. Yes, a fair amount of flat riding, but perhaps 15 miles of hill climbing (not counting descents).