You are perhaps confusing the sport of cycling with cycling-the-mode-of-transport. When I lived abroad, it seemed very difficult for people to understand the nuance, and difficult to see how missing the nuance was holding back bike-usage in commuting.
In NL, the average is ~20kph for muscle-powered bikes on bikes like [1]. Sports bikes like [2] are not usually used in commutes and considered as dangerous as scooters if used at speeds of 20mph. E-bikes legislation takes 25kph as the limit above which the cyclist is required to follow certain extra rules, and of course muscle-powered cyclist reaching that speed should perhaps also be legislated accordingly. The reason it isn't is difficulty of enforcement, but there are people advocating a maximum speed for bike infrastructure [3], at 25pkh.
Maybe if your country is flat as a pancake. If there's hills, then yes, people really can hit 20mph, even with casual bikes.