Because traffic volume.

Autonomous cars won't panic and cause pileups.

The way to prevent pileups is not to assume that everything will work correctly, including every system on every car.

It's to assume that every single thing that could break, will break, and design enough resiliency in that you have to stack up as many failures as possible before things truly go off the rails.

So to do this without killing people, you add the new systems with the same safety thresholds, then gather quite a few years of statistics, then, slowly let things become more efficient if that's both warranted by data and also warranted by simulation of events that you do not yet have data for.

Skipping steps in this because you want to put more cars on the road immediately is foolish and dangerous.