Each loophole has a base, people who care deeply about it. Against this is a vast population that doesn't even know it exists. Closing each loophole is an enormous political battle with very little payoff in tax revenue. If you add a new tax bracket or raise the capital gains tax, you get much more revenue for the same political lift. This is setting aside the marginal value of a dollar for the different populations taxed and the effect of economic inequality on corruption, democratic institutions, and social cohesion.