To over simplify the process: Budgets in the US are supposed to be revenue neutral. The use of sunset provisions, like the SALT cap, allow Congress to play with the math in order to make it follow its own rules. These provisions are really a gimmick because not extending them before expiration becomes a political problem. I.e. letting the SALT cap expire would “give the rich a tax break”. Note: I’m not arguing the validity of the SALT cap).