While sadism and stupidity may be two of the reasons for the US tax code, there are surely others. I also suggest extending special privileges to the rich without making that obvious to the poor, forging a weapon to use against political enemies, creating a huge number of bureaucratic jobs, benefiting a legislator's constituent state at the expense of the others, obfuscating the sources of government revenues, and as a means to launder dirty money into clean. There are almost certainly others.