The debt limit is something that was created by Congress. Congress creates the budget. The President is legally obligated to spend the money as seen fit by Congress. If the debt limit is raised it is because Congress does not balance the budget. This serves Congress in several ways. 1. It gives Congress leverage over the President (MAD), 2. It's something that Congress can blame on the President, 3. It's something that only Congress can fix. Trump, Obama, etc, and the Treasury Dept have no power over Congress' budget. They may make suggestions to Congress, but ultimately it is Congress who digs our financial hole.