It's the "having exhausted their resources already" which is the problem.
Suppose the IRS can audit a thousand small businesses and they recover more from this than their own costs. But at the same time most of the small businesses are innocent, and the audits collectively cost them several times as much as the IRS "profits". This is not a socially beneficial undertaking because the net costs across society exceed the net benefits, even if it has higher margins to the IRS than auditing large companies.
If you specifically want the IRS to target large companies then you can have them do that regardless of whether the margin of that to the IRS is less lucrative than the behavior that imposes more uncompensated costs on smaller businesses.
Maybe give them a collar and throw them a bone in the same legislation? I feel strongly like large companies have successfully lobbied and propagandised to conflate funding the IRS enough to effectually go after their rampant tax evasion with hurting small businesses.
It's not really all that hard to earmark a certain amount of IRS funds to only go after companies over XX size. I think this is actually essential under neoliberalism because one of the fatal flaws of neoliberalism is giving large companies more wealth & power and then expecting to be able to tax that back to fund the welfare state, which generally falls on its face as you've just given large companies all the wealth and power in the world to stop that from even happening. If neoliberalism is to survive as a political ideology and for us to not end up adopting socialism (which is bureaucratic and corrupt and inefficient), it's sort of essential that organisations like the IRS have a decent amount of power and for them to direct that power at large institutions.
Suppose the IRS can audit a thousand small businesses and they recover more from this than their own costs. But at the same time most of the small businesses are innocent, and the audits collectively cost them several times as much as the IRS "profits". This is not a socially beneficial undertaking because the net costs across society exceed the net benefits, even if it has higher margins to the IRS than auditing large companies.
If you specifically want the IRS to target large companies then you can have them do that regardless of whether the margin of that to the IRS is less lucrative than the behavior that imposes more uncompensated costs on smaller businesses.