The IRS isn't doing this. Congress did it, because it's an accounting trick that lets them say something costs less than it actually does, because all of Congress' budgeting is on decade-long horizons, and they factor in theoretical wage/revenue/profit growth, so them front-loading the same tax receipts allows them to very easily lie about the actual cost of programs. They can pull 80% of the tax receipts from made up figures a decade from now into their government revenue calculations.