Here are some of those reasons.
$3 for every $1 is an extremely misleading way of putting it, as though the IRS is a business and everything is good as long as it's turning a profit. A better way of putting it is that collecting taxes through enforcement has ~33% overhead, which is high. By comparison, the overhead of normal IRS tax collection is a fraction of a percent.
It also only accounts for the government's side of the expense, not the cost (and stress) imposed on innocent taxpayers who get audited to no avail in order to catch the smaller number of people who were actually committing tax fraud, and who are almost always small business owners because they're the ones with unusual tax returns. So the actual cost is even higher and is paid by a group of innocent people who create a disproportionately large amount of economic value while being at high risk of becoming unviable when you impose large surprise costs and time commitments on them like that.
The IRS also prioritizes its enforcement to maximize effectiveness, which means they're already taking the low hanging fruit and increased enforcement would be at the margin rather than at the existing average. For the average to be at parity you would need to be doing a lot of audits that cost more than the revenue they generate in order to cancel out the revenue gained from the net-positive enforcement that is already occurring.
It's possible that we could actually raise government income by doing less enforcement, because $3 is the average. It would be a net gain to eliminate the ones where we're spending $1 to raise $.50 (or $0) as long as we keep the ones where we spend $1 to raise $20.
