There are several reasons and none of them are good. But if you had to pick one cause for all the mess, it would be the fact that politicians have worked with interest groups and lobbyists to create myriad deductions, credits, exclusions, preferences, exemptions, and other loopholes.
This is a great deal for the lobbyists, who get big fees. It’s a great scam for politicians, who get lots of contributions. And it’s a great outcome for interest groups, who benefit from back-door industrial policy that distorts the economy.
But it’s not great for the American people or the American economy.