It’s just like gearing, though. A group that does 3 Heroic bosses and never does Normal isn’t gonna improve their gearing much, either.
And frankly, it’s a key design for a while now that the first 3ish bosses of the next difficulty are easier than the last of the previous.
Somebody who’s 3/10 Heroic? That’s a Normal guild. They should be clearing Normal for gear. Somebody who’s 3/10 Mythic? That’s a Heroic guild. They should be clearing Heroic for gear. And generally, people do.
(Or they just do M+ which is an easier, faster, and better substitute anyway. Which is the real problem for raiding guilds, these days.)