It also causes folks in one tax bracket to keep their income down for fear of paying way more. I've had conversations with multiple people who were deadly afraid of earning another dollar because they thought their entire income would be taxed at that rate. They were only disabused of the notion after bringing up the IRS tax tables and calculating it all.

Also the United States tax code is amazingly complex.

Yes and simplifying it involves in pissing off a lot of people. Closing loopholes makes people made. People who have money. People who will spend money to keep things the way they are. And (sometimes) we are those people.

I don't know how we can ever actually simplify the tax code.