Well I live in London and have for 20 years, so I've been to a few cities and average suburbs in the UK.
My point was that it's not just about population density. I'm sure cost is a factor, but it's not anywhere near the only one. The reddit posts go into some detail about the kinds of issues you can encounter.
We have a house in the US, not in CA, in a small rural town, and found out, when doing some landscaping (that actually included burying power lines) that about two feet under the soil, the entire ledge was granite. It took nearly two weeks of specialist diggers to put the tunnels through one property. It also required getting the town to give permission to close the road and take part of it out. We paid to have it re-paved and for the teams to inspect to make sure there was no damage to the other systems that run nearby (sewer, water, gas). Just coordinating the various different services was ridiculous. There is absolutely no way you could do that for entire densely packed urban areas, not to mention the potential damage to existing infrastructure for the kind of digging and tunnelling necessary.
I will also say that I've lived in 3 neighbourhoods in central London over the years and in every single one, our road and nearby roads seem to have required digging up fairly regularly, at great inconvenience. Our generally quiet road is clogged with standing traffic as we speak because National Grid and Thames Water have diverted traffic off the main road with a dig (once again).
I'm not arguing that it's not ultimately a better system. I am saying it's easy to sit behind a keyboard and announce confidently what should be done when you don't have to deal with the realities of doing it.
Have you been to Lahaina? The houses there were no more closer together than the average town in England. But we don’t have power lines swinging around where people are walking along the pavements.
I'm sorry, but I don't understand what you're saying here? The infrastructure of Lahaina is nothing like the infrastructure of LA.