My good friend is an engineer whose job has been to work on transmission line construction projects for several utilities including our local one. He just finished testifying at a court case because the utility was being sued by a group of homeowners for running the lines above ground rather than burying them.

His testimony was based around the fact that on average the cost to bury lines is roughly 10x more expensive than running them above ground, and he has had this experience across multiple different projects.

So when people say "it's expensive" they are literally referring to a factor of 10x cost increase. Maybe it's worth it in some cases compared to the costs of power outages and fires, but understand that we're not talking about a "slight increase" here, and that the people making these decisions are generally highly trained and experienced subject matter experts who are already doing cost/benefit analyses to take the best course of action that they can.