As noted, Bill Goldberg’s contract with WWE expired at WrestleMania 33. WWE still has the rights to sell his merchandise despite Goldberg no longer being employed by the company. Management is not in a hurry to sign him to a new deal because his asking pay is too much, and they know no other company can match what they paid for his recent run.
On a related note to Goldberg’s run with WWE, it is being said that Brock Lesnar had a huge influence to his return. Lesnar went to bat for Goldberg so that he can help him look strong. Goldberg was fine with his feud as he knew the creative team would not mess up their program because of Lesnar’s power backstage.