Let's try on an example: All revenue is taxed at 20%. Now this will work for some businesses, as the cost to make the product is low enough, and the sale price high enough, that 20% can be paid and there is still profit left over. But then there are very low margin, high volume businesses. They may have only a marginal profit per unit sold due to the nature of what they are selling, leaving no room for 20% tax.

So we add in an exception that says "You can deduct from revenue the cost of goods sold" (i.e., tax on net instead of gross). Now all the business has to do is find ways of hiding profit in the cost of goods sold.

Then there are products which contain materials that changed hands many times before it got to the final product. So if a raw material (lets say, cotton) is the majority expense, it has to go to one company to refine it, a second company to turn it into thread, a third to add color, a fourth to make it into fabric, and finally a fifth company to make a shirt. So that cotton gets taxed 5 times before it gets sold to the end consumer.

So let's add another exception, that goods sold for use in making another product aren't taxed -- that way, large businesses that produce the entire vertical stack in-house don't have an unfair tax advantage against smaller companies. Gee, now we have another loophole that can be creatively exploited.