There are a couple of things going on here. The first is beer, I don't mean to offend, someone check my math.
First - CAPUC sets what public power companies can charge, and everything they must do; including the "acceptable color" or type of bolt (literally - by bureaucrats with no experience) for transmission tower materials. If "public power companies" are not safe it is the responsibility of the PUC, and under their level of authoritarian control if utilities cannot afford to become safe it is the responsibility of the CAPUC.
@gwheihr - true, with the focus on things like tower color regulation, and lack of safety regulation W/O a reflection in pricing it seems like a CAPUC issue. ~$5B sounds like a lot without scale.
@AC - Power-lines are routinely buried everywhere in the world, but voltage and distance matter. A major road or business intersection for a 115kv line in SoCal might be $500k or less and often paid for by the requesting entity.
$40/foot maybe what it costs to underground a house from a local pole. Recently I got quotes from 3 different cable co. engineers to bring CABLE to my house (semi-rural) and it was 10k/.25mi. Or $40k/mi ($7.6/ft) for no-voltage existing overhead infrastructure.
@amimojo - Undergounding an ~220kv line was about $6M/mi 25 years ago, it's about that per-phase now so the ~$20M/mi is in the ballpark. Edison has about 12.6k miles of transmission lines over 50K/mi^2 - best case the cost would be ~$250B. PG&E has ~18.5k/mi of transmission lines across 70K/mi^2 or ~$370B to underground. Best case the entire PG&E $5B yearly profit would underground ~1.35% of their transmission lines/yr, or about 75 years to complete with zero profit, scheduling delays or cost over-runs.
@ravenshrike - Aerial insulation length/zones/costs increase with wind (and airborne dirt) but decrease with humidity. Humidity encourages dirt to stick to insulators and current to track up the insulators to the tower and increase flash-over (spark). Wind lowers humidity, reducing insulation length, but also decreases conductor to tower distance and increases likelihood of conductor to tower flash over. It also reduces humidity and brush dryness, increasing fire danger; it's not a simple sliding scale.
Not shilling for SCE; PG&E and SDG&E are problematic, that may have to do with NG infrastructure costs - I have no idea. Additionally, the CAPUC does regulate what they (non-municipality power companies) can charge; hopefully the above plus what they get for natural gas, $5B is their overall profit, not just from electricity etc. illustrates what a small profit that is for per mile electric infrastructure.
A separate discussion is that power from Hoover/Boulder dam is "for the pubic good" and to LA-DWP. They resell it to their customers over priced and kick back ~$.25B/yr to the city of L.A. If that power is a public good, maybe L.A shouldn't get that federal good for free.