One, they could be leaving the milder stuff as a plausible deniability thing. Like, “Of course we aren’t steamrolling the negative stuff, look how much is there!” while quietly removing the worse stuff.
Two, and this one is my favorite, with the mods on bad terms with the admins, the admins ironically just may not have the tools to keep up with the rest of the community and are settling for just erasing the worst stuff.
Also, I saw a theory yesterday that bringing back r/place could be a baiting tactic to take names and quietly ban the users who still have ill will towards the admins. If that’s the case, they may be leaving that stuff up intentionally and taking names…