Antonio Brown wants a new deal because he’s outperformed the one he currently has 3 years left on. The Steelers, on the other hand, have no incentive to give a new deal to a player they took a chance on three years ago by rewarding him with a huge extension they had no urgency to give him at that time.
There are arguments for and against extending him.
Against because it sets a bad precedent for extending guys early by encouraging others to attempt renegotiation if they feel they have outperformed their deal. There is also a growing sense that the proliferation of pro ready WRs that have hit the league the past couple of drafts is a trend that will continue, thus making WRs a fungible commodity and diminishing the value of the position as a whole.