1. Taxes inevitably get more complicated.As they say, "Loopholes beget more loopholes". For more details, listen to, "How The Burrito Became A Sandwich".
2. The more complicated the tax, the harder people will work to avoid it.
3. When taxes get complicated enough, the only people who are both motivated and capable of following the ins and out of tax policies are the professionals (e.g., tax lawyers).