There’s a big difference between burying distribution lines and transmission lines. It’s transmission lines that have caused the most destructive fires in California and those are indeed too expensive to bury.
Consider the fact that we lay oceanic cables.
It costs $1 million per mile (roughly) to bury transmission lines. PG&E has about 18,000 miles of transmission lines. That's about $18 billion to bury the lines. Sounds like a lot, but...
...the estimate price of these multi-day blackouts is on the order of $1 billion per blackout. Given multiple blackouts per year, and we're talking just a few years to pay back the price of burying the transmission lines. https://www.sfchronicle.com/business/article/A-cool-billion-...
But that's not the cost to PG&E. PG&E (or the state) would have to find a way to get some of that somewhat hidden opportunity cost.