That’s a feature accommodating realities of driving on public roads, not a bug.
If you drive on a 60mph speed limit highway, no one is driving 60mph, everyone is going around 70mph. If you decide to use autopilot and it limits you to 60mph, you singlehandedly start disrupting the flow of traffic (that goes 70mph) and end up becoming an increased danger to yourself and others.
Not even mentioning cases when the speed limits change overnight or the map data is outdated or if a physical sign is unreadable.
Over the road trucks often have speed governors, some companies limit their trucks to 60 mph because it saves a lot of fuel and leads to a much (50%) lower risk of collisions.
Apples to oranges. Stopping distance of a 16-wheeler is magnitudes larger than that of a typical sedan, so in their case it makes sense.
For specific numbers (after subtracting reaction distance being the same for both):
55mph: car 165ft, 16-wheeler 225ft.
65mph: car 245ft, 16-wheeler 454ft.
As you can see, the gap between a car's stopping distance and a 16-wheeler's stopping distance increases with speed increasing, and non-linearly at that. Not even mentioning the destructive potential of a car vs. a 16-wheeler.
I would agree with your point if majority of the roads were occupied by 16-wheelers, but it isn't the case (at least in the metro area that I commute to work in).
Note: I agree that it would be safer if everyone drove the exact speed limit, as opposed to everyone going 10mph above the speed limit. However, in a situation where everyone is driving 10mph above the speed limit, you are creating a more dangerous situation by driving 10mph slower instead of driving 10mph above like everyone else.
If you drive on a 60mph speed limit highway, no one is driving 60mph, everyone is going around 70mph. If you decide to use autopilot and it limits you to 60mph, you singlehandedly start disrupting the flow of traffic (that goes 70mph) and end up becoming an increased danger to yourself and others.
Not even mentioning cases when the speed limits change overnight or the map data is outdated or if a physical sign is unreadable.