Perhaps because it’s in their interest not to?
But the truth is, the filibuster and the supermajority are, quite likely, unconstitutional.
In the end, the Constitution prescribed six instances in which Congress would require more than a majority vote: impeaching the president, expelling members, overriding a presidential veto of a bill or order, ratifying treaties and amending the Constitution. And as Bondurant writes, “The Framers were aware of the established rule of construction, expressio unius est exclusio alterius, and that by adopting these six exceptions to the principle of majority rule, they were excluding other exceptions.” By contrast, in the Bill of Rights, the Founders were careful to state that “the enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.”
It’s high time we end this absurd and antiquated practice.