You don't have to dig up the roads to fix buried power lines any more than you have to tear up your walls to replace power lines in your house: you install a conduit (basically a pipe) under the road once and if the cable somehow gets damaged and needs to be replaced you can just run new cable through the existing conduit by simply pushing it in on one end and pulling from the other.

Transformers and other non-cable equipment are typically housed aboveground in little boxes or built in to the house, so they're actually easier to maintain than if they were installed aboveground on a pole since you don't need a cherrypicker to access it.

Obviously in a less wealthy small town with existing overhead infrastructure it doesn't make much sense to move it all underground "just because", but if you're already trenching under the road to install water/sewage/gas mains, it won't cost much extra to throw down an additional one or two smaller conduits for running power cables or telephone/cable/fiber lines.