And while it seems like most of the causes for outages would be removed by going underground, you only exchange one set of causes for another. Cars get replaced by gophers. Trees on the line get replace by ice when conduit riser fills with water and freezes. Aerial teardowns due to heavy equipment become backhoes that dig on the wrong place and tear up a line.
You also have confined space issues. Now instead of everyone being certified for climbing, bucket trucks, and cranes, they also have to add confined space training and equipment (including continuing education), confined space rescue teams, interlocal or interagency agreements for CSRTs. All of these things add hidden (but not cheap) costs.
In addition fault locating and repair on an underground is much more manpower and technology intensive than aerial. All of these things drive the cost.
Another problem that factors in is environmental regulations. It is much easier (and cheaper by hundreds of thousands of dollars) to get permission to do an aerial build than underground. Every underground build had to deal with permitting for aquifer contamination, native artifacts, wetlands remediation, and so on and so forth. Permitting can add 10% to 100% to a segment of utility infrastructure.
It all boils down to costs. If you can go to your local utility board, commission, or shareholder meeting and convince them that raising rates by 50% to 500% won't get them burned out of their homes, I'll bet they would jump at it. Every utility I know would love to move most of their aging infrastructure underground.