"consciously or not"
I guess this is at the heart of where I'm losing you. If despite intentionally designing a game that has a quite varied array of platonic sessions that aren't easily coalesced into one narrative focus I'm not necessarily making something that needs more narrative focus to consistently deliver the intended experience or an otherwise engaging experience.
"When you say you're building games that provide tools for players to do interesting things, the particulars of those interesting things are what make up the fundamental grounding of your platonic session (and the needed guidelines for narrative tools)."
Suppose I have my session concept. The game is then not in the concept but in how the tools I provide lure players through the messy thicket of the game's ideas and their desires and their ideas of what the game's ideas are. Players often crash out of that thicket in a whild direction, so some games try to cultivate a very specific and consistent path through it.
But (whether or not this or that designer had a Platonic Session in mind) I think it's interesting to give players instead an explosive energy and a bunch of spare parts and, if they must be mundane, a machete and see what sort of path out of the thicket they come up with and what sort of story is produced. Some game systems seem very good at bottling the sort of energy that makes that work rather than merely creating a directionless mess of themes players have to bring their own energy to.
Accordingly, I don't think it's a widespread design failing that mostly shorter, more focused storytelling games provide these path-cultivation tools in a way that fits your framework of a Platonic Session. That's less a mistake on the part of everyone else and a particular feature of those types of games! I think there's a lot to be said for games that consistently create worthwhile experiences but do not consistently create the same kinds of stories. We can still call that a platonic session, saying for example that in Game A, the platonic session is one that deals with themes B, C and D. But at that point we may as well be talking about themes, surely, rather proposing that too many games lack narrative tools.
Edit: I'm sorry, I got lost editing that when I went in for typos. I'm not sure how to revert and I'll try not to do that again but it's quite different from the post that was responded to below. Should convey the same ideas though.