When we're talking damage to a sensitive environment an ecologist would have the opposite answer to yours. If you dig up a natural environment you can't "plant seed" to reclaim it.
Not doing anything might indeed be better for the environment alone in isolation, but has to be balanced with other considerations, like the benefits of a power cable carrying clean power from wind farms to replace coal.
It is also entirely possible to plant native, locally appropriate seeds in a reasonably natural fashion. I am obviously not proposing a layer of lawn sod.
I think that's a question for the ecologists.
My guess, though, would be that part of the concern is people visiting the site on an ongoing basis as a tourist location, which won't happen with a buried power line.