Because changing the definition would require getting two diametrically opposed parties to cooperate, one of whom weaponizes the debt ceiling whenever it wants to do something inhumane, racist, or just plain stupid.
And the other is run by an oligarchy that makes its money shipping of US$ to the rest of the world for their own enrichment while pretending to care for the poor and the workers. And also engaging in racism by exporting abortion culture to the developing world so that there can be fewer brown people.