Imagine the cost of getting autorization for and digging a long hole through mile upon mile of in-city streets, and then repairing said hole - all while taking care of all the disruptions you cause to traffic and more. Unless you can find and use existing pipes in the right places with spare capacity (you're not going to, don't worry).
Directional boring is one of the most expensive ways to lay cable. Just going underneath at standard two lane road in a residential neighborhood is something like $50-$100K.
Undergrounding utilities (esp in a metro area) runs something like $250K to $1M/mile. There are many, many variables that affect cost.
Underground cables are also much more susceptible to failure when compared with aerial cables. When they do fail, it's sometimes more costly to repair them if they are buried as opposed to attached to a pole.
Underground utilities have do deal with more intrusions from water, ice, salt, digging, backhoes, insects, rodents and other stuff you find at "ground level." Ask any outside plant engineer and they will tell you that cost goes up and reliability suffers when you place utilities underground.
Aerial utilities primarily only have to deal with trees and ice. Oh, and gunshots. In the backwoods, the local yokels will sometimes shoot at transmission towers, taking out the fiber links with them.