Unfortunately, at this point, the number of accountants/tax-lawyers dependent on tax-complexity for their livelihood does create a pressure against simplification. Their and their clients' lobbying creates plausible-sounding rationales to keep every advantageous wrinkle in the code.

One particularly vivid example is Intuit's lobbying against federal and state efforts to make individual tax-filing easier:

http://www.techdirt.com/articles/20100723/09055310339.shtml