Because they owe him a ton of money, still, and could roll that forward to repair other problems coming next off-season. Jimmy looks like a placeholder on a team that isn't going anywhere this year unless Kittle gets back from IR quick. He's out of favor with the fans, and clearly unhappy with his treatment, as he should be.
The sooner they get rid of him, the more $$$ they save, the happier he'll be, and the better off they'll be. At this point, they're just throwing good money after bad.
Unless, of course, they think that Jimmy can help them climb back into contention this year (at 2-4, they're a couple of places outside that last wildcard slot in the NFC, and they've got some iron coming up in their schedule in that brutal division).