because unless the driver is a raging asshole usually the person behind while you are going 80 has more stopping distance than 5 fucking feet.

That's because you're thinking of human drivers whom are so slow to react that it is recommended they keep a whopping 2 or 3 seconds [0] worth of trailing distance to respond to changing road conditions like the person in front of you slamming on their breaks. The hypothetical clustering of self driving cars requires them to react at much faster times so they don't need to keep as much space between them.

[0]: https://en.wikipedia.org/wiki/Two-second_rule

Bigger. Faster.