Insulation is expensive. Furthermore, with AC you have to worry about the impedance of whatever you use as insulation. Basically the whole wire starts to act as a big capacitor, leaching power. (1)

I found some cost estimates: "The estimated cost for constructing underground transmission lines ranges from 4 to 14 times more expensive than overhead lines of the same voltage and same distance. A typical new 69 kV overhead single-circuit transmission line costs approximately $285,000 per mile as opposed to $1.5 million per mile for a new 69 kV underground line (without the terminals). A new 138 kV overhead line costs approximately $390,000 per mile as opposed to $2 million per mile for underground (without the terminals)." (2, page 17) Also, diagnosing and correcting faults is much easier above the ground than under, and you'd be surprised how often jackasses with backhoes take out underground infrastructure.

[1] https://en.wikipedia.org/wiki/Characteristic_impedance

[2] https://psc.wi.gov/Documents/Under%20Ground%20Transmission.p...

It is an order of magnitude more expensive to bury utility lines under ground, both in upfront costs and ongoing maintenance costs. This is due to the safety that has to be put in place to protect the wires, and the people who may dig into the wires while they are underground. Additionally making alterations to a power system due to a growing or changing residential power needs is much harder to do and takes longer when the power is routed under ground.

An individual pole is relatively fragile indeed, but they're stringed together with steel cables, so the whole assembly is pretty solid.

Source: Worked as a phone technician a while ago, climbed in these poles for a living. I hated climbing in "service poles" (individual poles not connected to the steel cable, used in cases where a house is far from the street), they'd wobble like crazy.

Underground power lines can get complicated in ways that are very expensive to remedy compared to aerial power lines. For example: https://www.jwz.org/blog/2002/11/engineering-pornography/

Most of Europe buried its LV power cabling decades ago. The answer is probably a mixture of "nobody bombed US cities", "no funding" and "lower population density". Although the latter doesn't apply to metro areas, obviously.

(Also, residential three phase power is fairly common except for the brits).

Florida has this solved.

Wood rots and breaks. Burial has flood issues. The solution is to install better poles.

Most commonly, the poles are concrete with a square cross-section. Some are round concrete, spun while hardening to allow a hollow interior. Some are galvanized steel, with perhaps a decagonal cross section.

One near my house is about 2 feet thick, with a label that says 50,000 pounds.

These are not fragile. :-)

We have also started using ductile iron poles. They are much easier to drill and last longer than the galvanized.

I think it's cost, mainly. Also, in my neighborhood of older homes it would be hard to get all the easements.

http://www.elp.com/articles/powergrid_international/print/vo...

Many of the easements for above ground utilities also contain provisions for underground utilities, so getting easements may be easier than you would think. However, having the easement, and having it be feasible to use are two different things. In my neighborhood, some sections have the utilities in line with the sidewalks and there is easy access; others have the utilities behind the homes, over many years many buildings have been built encroaching the easement. The encroaching buildings aren't legally there, but it would be messy to remove them to underground the utilities.

Also maintenance and access. I once had an underground power line to my apartment building fail, and it took the better part of the day for them to repair it. They had to tear up the parking lot in the process.

Road salt and underground vaults isn't a great combo.

At the micro scale: Toronto Hydro had to start a campaign of replacing its handwells or spraying then with an insulating coating because children and pets were getting shocked.

https://www.torontohydro.com/sites/electricsystem/Documents/...

At the macro scale:

Mmoke from vault fire: Nearby building closed for 6 months for decontamination

http://www.cbc.ca/news/canada/toronto/rbc-hydro-vault-fire-t...

Many newly built neighborhoods do indeed have everything underground. Retrofitting older neighborhoods generally isn't worth the cost.