Because of something called the franchise tag. Which the Bills can apply twice. So that means they can have him for 5 more years if they want.
I suspect that Peters and Parker have told the Bills the exact amount they want in a new deal ($10-11M a year and a huge signing bonus) and not to bother talking to them until the Bills agree to it. The Bills probably told him that at that amount, he'll have to play under his old contract this year and next year he'll get it. Hence the impasse. The Bills are ready to move on without Peters, but for Peters, it's not that easy. He's already out $430K in fines. Every game he misses, he loses $190K. And if he misses even one game, the Bills will go after his $1.5M signing bonus. Peters can't afford to have that happen and will eventually have to report.