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 ;)