It is extremely expensive and arduous to bury lines that have been aerial for decades. The cost to bury existing lines is exponentially greater than the cost to keep fixing them as needed for decades. New lines are sometimes buried depending on whether it’s an upgrade to existing infrastructure, power going to a new neighborhood, etc.
Do yourself a favor and look into exactly what it would take just to get approval from the city, county, etc and right of way from the various owners of land where you want to bury them. Not to mention the hell that would cause for traffic when you have to shut down roads to go under them or worst case tear them up and replace them.
I am not giving the utility companies a pass here but it’s not as easy as some people seem to think it is.
But still. For them to be large enough to be found means thats still a lot of people.