What would make the optimal reward scale, from Clearers to the plain-ol' Daily player

[Arashmin]

Well...in my days, the difference in reward was: -

No really. You got the exact same things regardless of whether you chose Very Easy or Infernal as difficulty. Or whether you set the difficulty bar to 1 or 100 in games with a slider. Later on (especially on Steam), you got a different achievement for clearing each difficulty, if at all. And that was it.

It's a concept that, for some odd reason, works for millions of single-player games. Modern titles as well. Yet in MMOs, it seems to be unfathomable.

Er, I don't think that was... ever the case? Unless you're talking about games that just have linear gear and just more difficulty modes, like old-school or even more recent single player JRPGs, although that's not even a hard/fast rule as a handful had aspects or additional unlocks, especially as you moved up into the PS2 and more modern systems. But even the earliest of MMOs like Diablo had better gear as you progressed up in difficulty.