berkeman said:
It would be lines that are at tree-height that are the problem
It's not just the height. In significant events it is more related to proper maintenance, things like failed insulators (supports) or insufficient tree trimming. There is also the question of design and construction costs. Most power lines are uninsulated wires, so if they touch a tree branch, bad stuff happens. So there are ongoing projects to convert to insulated wires.
There are options beside underground, which frankly isn't practical for such a large grid. There are many other ways to improve overhead transmission (hardening, in industry jargon). What should be done depends greatly on the local environment. Yes, overhead transmission has problems, but given the huge size of the network, the fact that fires aren't started every day, everywhere, is evidence that the current system isn't completely inappropriate. It just needs to be designed AND MAINTAINED properly.
In addition to the large cost involved, there is a big issue with how much work (i.e. time) it would take to make system wide change,
this article says at the current rate of funding, it would take about 1000 years to convert all of California's grid to underground and would cost each customer $15K. Frankly any "one size fits all" solution is hopelessly naïve. What is needed is infrastructure funding to maintain, improve, or convert, the existing grid in flexible and location specific ways. The fundamental problem isn't very different than bridge or pipeline infrastructure maintenance issues; we don't spend enough to keep up what we've built over the years.
It's not conducive to social media kibitzing. But the solution is to implement what the engineers at PGE, SCE, SDGE, etc. already know should be done. Unfortunately management, political, and economic issues get in the way. The engineers aren't so good at solving those problems.