Grades are mode specific, so requirements to get an S in aram on a particular champ are different than the requirements to get an S on the same champ in any given role on SR. A side effect of that tends to be cs scores are expected to be quite low for most champs, especially compared to their SR counterparts.
The tricksy thing is since grades compare you to everyone on the same champ in that mode, the best performances end up being the ones where someone just happened to get a team comp that really synergises with the champion and/or the enemy comp doesn't offer much to counter them, so more typical team comp match ups don't lend themselves to S ranks.
That said, I don't find it's nearly as hard as you're making it out to be. I pretty reliably use aram as my method of getting my weekly S chests, as well as often using it as my win of the day method. Just looking at my recent match history in the client, I have 2 ARAM S ranks in my past 20 games (one S- and one S+), so either I'm consistently exceptionally lucky, or they're not as unreasonable as you make out.