Possible theory that might be very wrong
>!Idk if this may sound stupid or anything since I've never really loved following up theories on shows except for gachiakuta.
This is all speculation, but we can definitely assume that one of the probable causes that the ground has a lot of erased knowledge about it's past nor the connection of the ground to the sphere is caused by the book of the watchman series ( It seem to be the only plausible instrument to erase knowledge and I genuinely believe that the book also has the ability not only to erase memories but erase written text as well. I mean if it was Canis who used them, the extent of the instrument's power must have been like really op since he's the original user.)
It has left me questioning though that if he wanted to erase all that history, wouldn't he had wanted the murals done by Macaca Icol gone? Like what Corvus says, the murals are kind of the "pure evidences" of the past. I'm guessing everything at this point, but these group of friends probably had a falling out or misunderstandins with one another. We still are unsure as to what do groundlings DO KNOW as to what they don't ( like the existence of Gods). We havent seen nor understand much of how much history are conveyed through the murals either.
It is also very possible that Canis did erase the texts about the ground and sphere as a "sacrifice" or "last resort" sort of thing, and Macaca wanted to preserve them in hopes that someone will be able to actually solve the mystery or the problem they faced.
I've seen a couple theories now about how Corvus could be one of the friends of Macaca (mostly because of the eye symbol looking similar to his tattoo and the possibility of him being kuro) But he also suggests that the sole existence of the cleaners was to help uncover the grounds mysteries and also to help his friend that we still dont really know of.
I probably didn't explain this properly and might be overly assuming and misunderstood everything. I am excited tho to see more of how the story moves forward. !<