I think the simplest explanation is also the best one. Like you said, it is very difficult to tax wealth.

I think we have no option other than taxing loans and other money movements like that in sufficiently large scale as ordinary income. If I get a loan for USD 200k for a house once a year, I think it isn't income but if ElMo gets loans worth USD 20M a year, every year, he should pay income tax on all of that as if it was ordinary income. How he pays it? I don't care. Sell some assets. Oh and that sale is also taxable.