The IRS can indeed go after rich tax cheats. Under the Trump administration, they have deliberately changed their priorities away from auditing wealthy people and toward auditing poor people. This is something that can be changed with by executive order without any need of Congress.
The reason the IRS is targeting poors is that their tax returns take less effort to audit and thus on purely fiscal grounds, it's the most effective use of their limited budget. To go after more of the more complicated returns will require Congressional action to restore funding for audits.