Well Coughlin is pretty well entrenched there for a while, being a pretty consistent coach throughout the years with several playoffs appearances under his belt (not to mention a Super Bowl win). In addition, Bill Cowher requires the freedom to bring in all his own people, which I don't think the Giants' owners would like very much. He's a great coach, but he just wouldn't be a good fit with the Giants.
Also lol @ your suggestion that the Browns will EVER win a Super Bowl :D