Is this referring to average speed? I suspect that this doesn't take hills into account. In my rides with a loaded trailer, my usual average speed is about 11 to 13 mph and my top speed on downhill sections ranges from 20 to 25 mph. My uphill speed is about 5 to 7 mph.
Also, 20 mph downhill and 5 mph uphill gives an average speed of 8 mph, a bit lower than your claimed speed. I think that’s the generic pattern for hills: you lose more on the ascent than you gain on the descent because you spend more time on the ascent than on the descent.
Also if I were to maintain an average speed of 8 mph, then my commute where I pick up my kids in different locations would take me nearly 2 hours as opposed to about 1 hour and 15 minutes. So speed matters if you want to complete the ride in a reasonable period of time.
The EU seems to concur with that. https://en.30kmh.eu/why-30kmh-20-mph/trendsetter-cities-for-...: About 50% of Dutch road network are having 30 km/h speed limits.
Also, typical cyclists speed inside city centers is more like 8 miles an hour, the speed of a reasonably fast runner (https://bicycledutch.wordpress.com/2018/01/02/dutch-cycling-... says 12.4 km/h)