Because 19mph is easy, ya know? But god, I wish it made a difference bigger than about 7%. Then I could burn more, eat more, and cover less distance. A 100km ride with typical elevation (~35ft/mi) will take me ~2,200kcal, whether it averages 16mph, 17mph, or 20mph. The only way to burn more calories is to ride more effective distance-- and I'd rather go real fast on flat ground than sweat up a mountain really slowly. At least most of the time.