The US does go after the wealth for tax fraud. I think the bigger issue is that the wealth pre-emptively hire CPAs/lawyers to take maximum advantage of tax rules in a way that may not be prohibited, innovating faster than legislators can act.
The IRS may be underfunded, but it's misleading to say the IRS does not go after the wealthy for tax fraud. The chart in that article shows $1M+ earners to have the highest audit rate, albeit some of it correspondence audit, and says 25% of audit man hours are spent on millionaires.