That's because the bills those committees report then go to the budget committees and get wrapped into a reconciliation bill. The Budget Act gives reconciliation bills procedural protections against amendments and filibuster on the Senate floor. Scoring CBO is required to develop a cost estimate for virtually every bill reported by Congressional committees to show how it would affect spending or revenues over the next five years or more. Some examples of popular tax expenditures include the accelerated depreciation of machinery and equipment, and the earned income tax credit. The Budget Act requires Treasury to publish a list of tax expenditures each year.
Tax expenditures are important because their net effect runs in the hundreds of billions of dollars.
There's no statutory definition of a tax expenditure.