There is a lot more to the economy than W2 income. If you want to tax worker compensation, you need to decide what is compensation. Is a nice office compensation? A company phone? A company car? A company apartment? Company-paid airline travel and hotels? Meals? Employee discount? The answer to most of these is "it depends."
If you want to tax business profit, you need the idea of business expense. Well, there are a lot of small business owners in this country. What is a business expense vs. a personal luxury for the owner?
If you want to tax asset appreciation, you need the idea of cost basis. If you don't want that to be arbitrarily cruel, you need the idea of capital loss, and capital loss carryover. Now you notice people are cheating, and you need the idea of wash sale.
The fact that complexity exists doesn't excuse the additional unnecessary complexity added at every step of the tax filing. Nor the fact that it can't be done online directly with the IRS, without potentially malicious third-parties involved.
If you want to tax business profit, you need the idea of business expense. Well, there are a lot of small business owners in this country. What is a business expense vs. a personal luxury for the owner?
If you want to tax asset appreciation, you need the idea of cost basis. If you don't want that to be arbitrarily cruel, you need the idea of capital loss, and capital loss carryover. Now you notice people are cheating, and you need the idea of wash sale.
A lot of this complexity is essential.