NFL teams have the franchise tag, which enables them to keep a player bound to them in perpetuity for extra seasons. NFL teams have also negotiated successfully with the NFLPA to get four-year rookie contracts in every round, including a fifth-year option for first-round picks. This is in a league where the average career doesn't make it more than four or five years.
Very few players are actually traded for draft picks, and restricted free agency is an oft-ignored market because teams are reticent to lose picks. Especially picks in the first few rounds.
The only leverage a star player actually has is holding out.