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).
Really? I had heard that because streets are regularly dug up (well, every 5-15 years or so), and because the cost of doing so is almost always _way_ more than the cost of what goes in there, that for this reason there's already tons of capacity going unused (so-called "dark fiber" -- the Wikipedia article on the topic shows an interesting history of revisions ... ).
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.
You don't have to take my word for it. The Telcos did their own reliability surveys.
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.
Hm interesting. Where I come from, the aerial power cables go down pretty regularly due to wind/trees from storms. I haven't heard of similar issues with the underground lines, but I don't know anyone at the utilities. I would have thought that glass fiber would be impervious to all but backhoes, though.
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).