Because he's still one of the best all around TE's in the league, not the best, but there is a reason that he literally never comes off the field for one of the most dynamic offenses in the league. Aside from his abilities as a receiver, we use him as an inline blocker and as a lead blocker out of the backfield at times.
As a receiver, his numbers last year were right in line with what he has always done in terms of YPC and TD's. The only difference is that his targets went down, which is basically the result of us running more and the fact that our offense has several weapons in the passing game with Dez obviously, as well as Williams as a capable 2 and Beasley developing into a very dangerous slot receiver.
He may not be worth 8 million a year, but 2014 Witten is still very valuable and worth at least about 6 a year, that difference is not worth losing a player of that caliber. If he drops off a cliff this year that is one thing, but if he maintains his level of play from last season, you're basically cutting off your nose to spite your face by trying to prove some philosophical point if you cut him.