The IRS would think that it is important to have an IRS audit on all tax returns but sometimes it is not worth the efforts and resources of an IRS audit. So, the IRS audits only questionable tax returns or tax returns that have been selected according to complex math formulas.
Without an IRS audit, the IRS will not know if a tax return is correct and legitimate. Sometimes, there are leads that a tax return if fraudulent or questionable and the IRS needs to investigate.
Not all IRS audits end badly. Sometimes, the person getting audited by the IRS can walk away with even more money than he or she claimed.