The major problem with the IRS is that is handicapped by law and politics from doing it's job. There's an insane amount of capital sitting in offshore accounts that probably should have been taxed. Loopholes for the big players are crazy.
If the law prevents the capital from being taxed, such as loopholes, then those laws are not handicapping the IRS in any way as the practice is perfectly legal. The IRS has no standing to do anything pertaining to that capital in the first place. If the capital should have been taxed and hasn't, then that implies law-breaking that's a bit beyond what involves just the IRS.
But you are correct about the politics, as it is politics that creates the loopholes to begin with. But that's a matter of annoyance, not illegality.
Tax loopholes are usually used in the context of money that was intended to be taxed but cannot be due to technical reasons, such as language of the law or inability for the IRS to execute enforcement of the tax.
If the loophole exists because of the law then the practice is perfectly legal and therefore that money was not intended to be taxed in the first place. In that case there's nothing to prevent the IRS from doing something because there's no reason for them to do anything at all.
If the IRS is unable to execute enforcement of the law concerning taxation then that's not a loophole, it's likely a criminal act. That would imply that something could possibly change in the future that would no longer prevent the IRS from collecting the tax, with penalties.
I see these two as completely separate things.
People need to get out of this mindset that the big companies are using loopholes in the tax code to cheat the government out of tax revenue. If the loophole exists due to the law then there is no cheating, just bad law. Public companies are practically obligated to reduce their tax liabilities as much as possible and if the government gives them the abilities to do so, then why is that the company is considered the evil entity in the matter? They are simply doing exactly what the government and the law has told them they are obligated to do.
The IRS has been put in an impossible position by the current state of election laws, which allow the creation of a variety of different types of special political organizations, each of which has different disclosure, tax, and spending restrictions.