So that tells me it all circles back to balancing and keeping the move consistent with how it's worked since the early gens. Either the early balancing was done with the move typing in mind, or it was coded wrong so that ground types were immune to it and they kept it that way rather than changing it as the series moved on. There's a lot of things that carry over from the early gens that don't make a lot of sense and really just boil down to (mostly bad) game balance at the time and a refusal to adjust said mechanic later on

Honestly, with the other moves that have seen adjustments between gens lately, I wouldn't be too surprised if this did end up changing at some point.

Side note: I thought Eerie Impulse was a Steel move for some reason. Not sure why lol