This is very valuable information if true, can you provide some sort of references?
Not to mention that above ground lines have slack between poles while underground lines are pulled relatively taut. Any shifting in the ground is going to result in a break underground before a break above ground. Above ground lines are also better founded because the poles are sitting on foundations or driven deep so their ability to resist ground motion is a bit better while electric lines move with the ground around them pretty much in sync. This is particularly important in seismically active areas like CA but underground infrastructure actually typically performs better in actual seismic events.
While, yes, the cost to dig up a line is likely to be higher than the cost to repair a hanging line, that's just one cost and consideration. Economic loss due to wildfire as well as rolling brownouts to try and avoid wildfire are both costs you'd need to consider. You also have the cost to maintain vegetation around powerlines, vs basically no pre-emptive maintenance on buried lines (and little to no weathering).
And you, of course, have differing likelihoods of the line breaking in any case, which means the per-fix cost is only part of that picture. Lots of potential reasons, but I'd be curious to see a full study on those reasons for both above ground and below ground, their potential solutions, and their potential costs, but without that you have a very fuzzy picture.
"lo barato siempre es más caro"
The cheap is always more expensive.
see: https://en.m.wikipedia.org/wiki/Time-domain_reflectometer