We all know that these companies are under-taxed - and over-use loopholes
Y'know, I am in general in favor of a strong regulatory state, and of relatively high tax rates for individuals.1 And I'm not fond of laissez-faire treatment of business, nor inclined to preach the virtues of the market.
However, I've spent a fair bit of time reviewing the economic arguments around business taxation, and I'm really not convinced corporations are under-taxed. Corporate taxes eventually have to fall on customers, employees, shareholders, and/or capital investment; there's no where else for that money to have gone.2 If it's not diverted by corporate tax, it will get taxed elsewhere,3 and in the meantime it will contribute to economic activity as it passes through more hands.
Frankly, I'm much more interested in seeing regulatory pressure for raising wages than regulatory pressure for increasing corporate tax enforcement. Put more money directly in the hands of the lower-paid employees. There's ample evidence to show that helps encourage economic activity broadly across a country in a fashion that reduces wealth inequity and immediately helps the people who need it the most.
As for loopholes, corporations arguably have a fiduciary duty to their shareholders to seek them; and even if they don't, they will have competitive pressure to do so - if their competitors find loopholes and retain additional working capital, those competitors can beat them on price. In my opinion, the real problem with tax loopholes isn't the moral hazard but that they're regressive: they're much easier for large organizations to use than for small ones to, and so they grant a competitive advantage to the existing large players.
1Here in the US, my wife and I pay a ridiculously small amount of income tax. Even when you add all taxes from all sources, taxation on people in our economic bracket is really low relative to other industrialized nations, and actually regressive, thanks to the cap on FICA.
2It can be stashed temporarily in tax havens, as many of the big-corp villains are criticized for doing, though the Trump tax changes made that less feasible for US corporations. But eventually that money has to go to one of the sinks, and in the meantime it loses value to inflation unless it's invested in something that returns better than investment, in which case it's really being invested, not hoarded.
3Of course not necessarily at the same rate. Here e.g. the difference between the nominal corporate rate (35%) and the long-term capital gains rate (12%) in the US is a striking example of this discrepancy. But that doesn't imply that more-aggressive taxation of corporations is the solution.