Ah, so Japan has massive dry forests, through which hundreds of lines of transmission lines are strung?
Perhaps the reason Japan doesn't have the same fire problems is that it has a much older history of population, and thus the forests there don't have the same level of old-growth wood, and they also haven't had a devastating drought hit those forests and the cities around them. Not to mention that California has had a population boom since a large number of those power lines were designed, and while capacity upgrades have been made, choices in routing suitable for, say, a singular 200KV line may not be optimal now that 800KV lines are possible. Meanwhile, Japan has been experiencing a population decline for decades now.
This problem is a lot more complicated than a surface level look would indicate.
You've got expansion of both loads and generation sites, including intermittent ones that then need more complicated switch gear, as solar installations mostly are on distribution circuits designed only for loads, as well as residential systems that aren't equally powering all phases of a circuit. That's a major technical problem, even ignoring the management and regulatory issues PG&E had.
Then add on the California droughts, and the massive amount of dead brush that resulted. Now even small sparks that would be harmless in say, Seattle, spark a massive wildfire. Plus the high temperatures that would raise the operating temperatures of the equipment further. That's an environmental issue. It isn't just PG&E driving that, it's global climates, as well as the massive population and general water use in California.
Compound that with the engagement and regulatory structure. Companies exist to make money. Utility regulators exist to keep them from making too much, at the expense of their customers. Any time a utility asks for a rate increase, for say, upgrades, the regulator gives it a close look if they care about doing their jobs. After all, at least in my area, the allowed profit is relative to the capital investments made, and allowing additional capital spend for upgrades, and the attendant rate increases, means more absolute cash that can be paid to investors. So regulators actually have an incentive to prevent upgrades, if they believe they are unneeded.
And just like that, you have a recipe where people are mostly doing their jobs properly, just not perfectly, and the evolution of risk factors screws a whole bunch of people, because plans with a 20+ year implementation time didn't properly account for the changes in the operation conditions.