A blanket pardon, unbounded pardon, issued without a backing legal theory would amount to overruling both the justice branch and the legislative branch.

District Attorneys, the FBI and other agency's responsible for prosecuting crimes report to the President as part of the executive. Each has broad discretionary power on which crimes they prosecute, what sentence they seek, and what deals get cut. The president having some authority to wave crimes goes along with this ( and also short-circuits the president manipulating law enforcement for their own purposes ).

If the president one day decided that he didn't want the executive branch to enforce the law and pardoned everyone... well they'd be impeached quickly, and the supreme court would almost certainly rule that the president doesn't have the power to pardon everyone. Assuredly an injunction would be issued to block the pardon coming into force.