A road has to be designed a certain way based on the speed. You can't just put up a speed limit sign, and speed limit signs are very often not based on what's safe.

You don't seem to be addressing the criticism of your argument, which is that urban speed limits are determined by likelihood of collision with another car, a bicycle, or a pedestrian in the event of an unexpected need to brake hard. This depends on more than just the physical engineering of the road.

There are lots roads with the same environment or with environments less likely to lead to collisions with cars, bicycles, and pedestrians that nevertheless get lower speed limits.

This isn't the case in New Zealand. It's normal for all rural roads in New Zealand to be posted with a 100 km/h limit, with speed recommendations (not limits) on the corners.

Even winding mountain passes, which wouldn't be possible to take at 100 km/h without a race car will still have a 100 km/h limit. It's up to the driver to make a judgement as to how fast they can safely drive.