If companies were taxed on their revenue rather than profit, it'd be an interesting world. I'd love to see a calculation of what would occur, but I'm guessing prices would go through the roof as every single step of the supply chain for every product would have to add 10-20% to cover the taxes.

Yes. And vertically integrated companies would have a huge advantage, making it really hard for smaller companies to compete.

In the US that is exactly how taxes are paid. But because most people are shit at record keeping it’s all bundled under what’s called the Standard Deduction. Though you can itemize if you like.

Because you don't need any of that for your business only.

Because that would make tax evasion substantially easier.