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.
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.