It's a combination of things, really. Underground is about twice as expensive as above-ground, harder to maintain and repair, and pretty much impossible to upgrade.

A lot of the medium-voltage stuff is already underground in some areas. All new 50kV-150kV stuff has been going underground for decades, and the stuff remaining above-ground just hasn't reached its replacement age yet. Despite its drawbacks, above-ground is unpopular enough that burying them is worth it.

However, the high-voltage stuff (220kV & 380kV) is still almost entirely above-ground. Those cables are extremely difficult to construct, and anything beyond a dozen miles or so simply isn't technically possible yet.

It's not going to cost 5-10x more to make the artistic design. And there are reasons beyond installation cost to avoid underground transmission -- maintenance is also way more expensive, and I believe it's also more fragile that way.

Depends on location. Underground costs more and is harder to repair.

So for earthquake prone areas, you will find above ground power pylons everywhere simply because that is safer and cheaper than the alternative.

Where disasters are unlikely, they tend to be underground.