Liability already gives them legal incentives to avoid incidents.

That’s quite optimistic, especially since there’s almost no enforcement of liability.

… open-weight models capable of doing all the things that we’re concerned about are already in the hands of bad actors around the world.

Computational resources are the limiting factor. AI companies have resources which no independent actor could ever dream of, which makes them far more dangerous to the general public. A chihuahua with an attitude problem can only do so much damage. A Great Dane with an attitude problem is a threat no one can afford to ignore.