Why the IRS is Deprived of Adequate Staff and Resources

Washington Post columnist E.J. Dionne recently noted that the IRS commissioner reported that as much as a trillion dollars a year is not collected because of errors and fraud, and because the IRS lacks the staff to go after the money. I also read that every dollar of extra funding for the IRS would yield six dollars in added dollars collected. This situation is self-worsening: The more people hear about how many people are getting away with cheating on their taxes, the more people say, “Why am I such a chump as to be paying the full amount?” 

Why doesn’t Congress act with alacrity to provide the IRS with adequate resources? Could it be that a lot of rich donors to political campaigns prefer to keep the IRA underfunded. What other reason could there be?