The only reason they can’t make a “regular” law is because they made a law spelling out the process. Their power is restricted because they passed a law in the past saying it was restricted.

As the only body capable of updating the text of the law they have ultimate power. That they chose to constrain it was an altruistic act of a sovereign body. But only they (and their predecessor the continental congress) had the power to restrict themselves. The courts can only hold them to the text they themselves wrote.