Or: Megacorps have entire teams of people looking for ways to reduce their taxes, many of which are legally dubious but the risk of being caught * the size of the fine means it makes business sense to do it regardless of legality.
"Legally dubious" is the problem, because ambiguous laws are supposed to be interpreted most favorably to the defendant rather than the government, and then all parties have to incur much higher costs because the ambiguity means it goes to litigation, and there is a significant chance that all of those resources are consumed and it comes out in favor of the corporation in the end. The IRS much prefers to find cases where the taxpayer is clearly violating the law.
Isn't this exactly what all megacorps are hoping for everyone thinks? I am not saying that you are wrong but these megacorps are some of the most evil the Civilization has ever seen (see Meta) and now you and I are hired as tax attorneys - pretty soon (if not right away) one of us will go "this shit's very much so illegal but who is actually going to audit us? - the answer, per your comment is basically no one because we think these megacorps and their lawyers are there to play by the book...
The general problem is this. You have a company with its headquarters in Ireland that designs a product in California, manufactures it in China and sells it in Germany. In which country did they make a profit and therefore owe taxes? It depends on what each subsidiary bought from the others and how much they paid, so they're going to structure their operations so that the profit ends up in the one with the lowest taxes. That's the defect in "corporate income tax" for international companies, and why it gives international companies an advantage over domestic ones.
In order to fix that you need a tax code that says the taxes have to be paid to the country where whatever subset of their operations you want to tax is actually present. But then it's not "corporate income tax" anymore. If you want to tax them in the location they have workers it's payroll tax, if it's where they have buildings it's property tax, if it's where they have customers it's VAT, etc. You need it to be something they can't so easily move out of your jurisdiction. Because if you say that it's profit then they'll just arrange to make their profits in Ireland or Bermuda.
Doesn't care that the citizens pay tax in whatever country they live in. If they earn over some 6 figure sum, they have to pay tax in the US as well.
That would put US corporations at a distinct disadvantage on the global scene, so it won't happen. Disadvantaging citizens doesn't seem to matter as much.
But how would that even work for a corporation? Suppose you did that; is anything multinational going to remain a US corporation? Of course not, they'll just register in some other country. The CEO of Stellantis nee Chrysler is in Michigan but how many people can guess which country the corporation is registered in without looking it up?
The citizen has literally upped and moved themselves entirely to a foreign country.
The corporation has just forked a bit of itself elsewhere.
And yet the corporation can't be taxed, but the individual can.
Now you've created a disadvantage for corporations to bid on government contracts, reducing competition and causing the government to pay more for stuff. Meanwhile the companies that actually bid are then the ones that specialize in lobbying the government and register locally and other corporations still register elsewhere.
> tariffs
If you were going to use that you could just as easily use VAT to begin with.
Only if the percentage of their business represented by US government contracts is more than the US corporate tax rate, i.e. only for companies like Lockheed whose business is focused on government contracts. But those are some of the largest "domestic companies" being put at a disadvantage by the existing tax system because they already can't use the same international tax avoidance strategies as other companies when they're required to use domestic supply chains by those same government contracts.
Meanwhile the companies that do lower percentages of their business with the government would just stop doing business with the government at all, causing the government to pay more for things because that company would otherwise have been the one to get the contract by being the one to offer the government the best price.