From an outsider's perspective like yours or mine, I see it as two things.
1. Arc has so hyperspecialized into story-rich h-games (a category I've personally never seen a jam for) that it would be relatively pointless to try.
2. Even if it's two days, things like the ear infection mentioned above and other unforeseen circumstances can delay progress on CK updates, thus leaving a different and likely lesser amount of time than allotted for making a game for a jam, either lowering quality of the jam game or of the CK update. Weekly updates are a hard thing to keep up with...
I can't speak for any other devs you follow, or even Arc for that matter, but this is my take on why.
Not sure why you're being downvoted - it's a reasonable question. EzraYugi covered the answer very thoroughly, but just so there's an official response:
1. Time spent on game jams is time not spent on working on the game I'm being paid by amazing supporters to make.
2. There are no game jams for my specific field, at least not ones that would provide anywhere near the necessary number of interested players that would make it worthwhile.
3. I'm on a weekly update schedule, which already kicks my ass (I get maybe part of Sunday off) so I'm not looking to add more work to my plate.
Those are the main reasons, though there are other, smaller ones. A good rule of thumb, when wondering why nobody does something, is to assume there's a good reason for it ;)
