Why is it that people can apparently drive at 30, 40, 50, 60, 70 mph instinctively, but 20 mph is impossible without either constantly staring at the speedometer or some sort of technological intervention?
Partly to due with the fact that people have been driving at 30-70 mph consistently for decades, with only 20mph for very short periods of time in high risk areas.
Partly because 20mph is so slow in comparison to all those other speed ranges you listed that it feels massively different in comparison to any of those other speeds.
Vehicles are so well built, and the driving experience and controls are so smooth, it means there's so much less concentration needed to take in the surroundings at 20mph it's actually distractingly slow for extended periods of time.
It may sound counterintuitive, but it's a real thing. It's why we've had a range of 30-70 for decades. To ensure speeds for the roads are appropriate. And 20mph just isn't appropriate for so many of the current zones that are being converted.
Additionally because it's more difficult to maintain a low speed within x percentage of that speed because every single mph difference is massively percentage difference of that speed compared to all those other speeds you listed.
The looking at the speedometer wouldn't actually be as much of an issue if it wasn't for the constant barrage of cameras to penalise us if we fluctuate a small amount. There's such a thing in motoring as going with the flow and driving appropriately to the rest of the traffic. You will find in these 20mph zones though, the vast majority of the traffic will easily creep to 25 +-3mph. This, combined with the camera make the speedo checking necessary.
Honestly, all these people harping on about 20mph is fine have only ever driven through school zones or only have a single street near them converted. Anyone with decades of varied driving experience will understand that the transition to these extended multi mile long 20mph zones is not based on the science of driving and is just reactionary and political.