By that logic, wouldn't people just not wavedash, and always just short hop-> fastfall land for general movement? I feel like wavelanding is more or less the same speed, because you get to the ground faster, even though it's 10 frames vs 4 frames for regular impact land.
I'm assuming you meant "waveland" and not "wavedash" there.
You waveland down onto platforms because it interrupts your upwards motion and can be done before the peak of your jump (unlike a fastfall), so when your jump would normally take you above it you can land before that, thereby minimising the time you're in the air before landing. Airdodging down then landing in this situation rather than just landing is slower, easier to react to, and doesn't maintain the threat of an aerial on the way down (which is why empty jumps work). There's no benefit to it.
It's also important to note when the reaction window for being sure the opponent is doing an empty jump really starts. Even if the overall length of each option is approximately the same, you have less time to recognise what's actually going on when they just land than when they airdodge down, since with the former it's ambiguous whether the opponent is going to do an aerial or not the entire time they're in the air until right before they land, and then once they do they have less lag than a waveland too, whereas with an airdodge you can tell immediately and they have more lag after landing.