Because the stockholders are the first line of defense. It's entirely likely that PG&E will end up reorganizing under a new corporate umbrella, where the stockholders simply get totally hosed. (That is, the company goes through bankruptcy and a new company takes over the operations, debt, and infrastructure. That's why I will wait to $5.
The legislation that CA passed does not cover PG&E completely by any means, it gives them a way to borrow more money safely and limits damages. It won't protect them in a truly disastrous situation, AFAIK.
Here's the kicker: PG&E just published that they have less than $2 billion in insurance. That's not even close to covering the claims that will come from the Camp Fire. That's why bankruptcy. PG&E doesn't have the money, and it is unlikely that it will be passed on to the ratepayers. That's why we have corporations, to shield from liability.
Can I ask why you'd buy at all? Because shares won't carry over in a reorganization, right? They're going to be sued into oblivion and will be uninsurable and the hourglass has already been flipped. Take a couple years maybe. They're going to completely rebrand.
Not that the replacement will be any better.