Roads commonly carry traffic at a different speeds than listed speed based on actual conditions, perception of conditions, condition and nature of vehicles and because speeds are often set inefficienctly.

Its not uncommon to see stretches with traffic constantly packed 4+ lanes going 75+ each way.

Trying to put the same burden of vehicles through at 65 with 300 ft between would be a joke. A given lane would carry less than half actual traffic. Bad traffic would become a city wide disaster.

Then consider that you can only impose this on a portion of drivers per year and drivers have to drive with those without governors indefinitely.

You also discount issues with safety by following at an appropriate distance. Trying to fall back 300 ft can only be achieved by driving too slow leading to both passing and people taking this new huge space whist giving you the finger leaving you with even less space.