I'm interested what would cause underground cables to break more often. Is it people not "calling before you dig" and accidentally severing them or is it something else?
Ground moves, especially in California. Sometimes it’s subsidence, sometimes landslides, sometimes it’s seismic related, sometimes it’s frost heave or the like. Properly studied and buried (with proper backfill) usually is protected, but big enough changes will cause problems.
Throw in backhoes, erosion, etc. along with it being more expensive to fix when it does break, and overall it isn’t always guaranteed buried will be cheaper maintenance wise.
The slightest crack will allow ground water in shorting out the wires. Cracks are somewhat common after a few years because the ground moves via the yearly freeze-thaw cycles we have up north.
It would be pretty ridiculous if a little water could short out a buried transmission line. Do you really think generations of engineers haven't figured out how to protect buried lines from ground water?
I don't know what PG&E will use in this case, but all buried power and transmission lines lines are robust against water infiltration. Solid-dielectric cables are rated for direct burial, and cooled lines are encased in steel conduit. Both types are sometimes further protected in concrete.
Engineers encase the cables in rubber or plastic, which degrades over time. Or steel pipes which rust over time. there are things we can do about this, but they just add time. When you only have a little bit of cable odds are it lasts, when you have thousands of km odds are there will be constant problems.
Code is for me to bury little tiny cables in my backyard 24" down. These giant mega cables have to be at least 36" down, right? Are rodents going 36" deep and eating into cables really a thing?
That's because they always remove the carcass. You're supposed to leave the carcass of the first one as a warning to all of the others.
Growing up, this was something that occurred multiple times a year. Squirrels would climb the poles and short themselves across the 2 leads on the transformer. Zap, one less squirrel, and several hours without power while the power company came out to diagnose the problem and then send out for a replacement.
Throw in backhoes, erosion, etc. along with it being more expensive to fix when it does break, and overall it isn’t always guaranteed buried will be cheaper maintenance wise.