In the NFL, teams are built through the draft. Franchise-altering players rarely ever hit the open market and you are more likely to improve your team by trading a player for a draft pick than signing one in the offseason. Successful teams like the Green Bay Packers have built the majority of their roster through the draft. The Dallas Cowboys have also learned that this route is the way to go.
The Cowboys offense is loaded with drafted players, as 10 out of the 11 starters were drafted by the team (fullback Rod Smith is the only player not drafted by Dallas).