NFL franchises are extremely profitable and rank as the highest valued teams in all of sports. The Collective Bargaining Agreement (CBA) attempts to maximize the league's value and profitability and share the wealth equitably between players and management by creating, among other things, free agency, a salary cap, and mandatory retirement benefits. However, the main CBA provisions along with a tradition of not guaranteeing yearly salary disadvantage young players and veteran reserves. Players are not eligible for true free agency until their fourth year in the league, dead money from signing bonus cap charges attributed to players no longer on the team limits what organizations can spend on reserves, and rising minimum salaries and retirement benefit contributions make many veterans too expensive for teams to sign. Although teams should prefer having experienced players as backups and on special teams, the NFL regulatory structure gives teams the incentive to sign young and inexperienced players to those roles.
Katz, David E., The Veteran Premium Problem and the Effects of the NFL
Collective Bargaining Agreement on the League's Reserves (April 3, 2008). Available at SSRN: http://ssrn.com/abstract=1116244 or http://dx.doi.org/10.2139/ssrn.1116244