Over what timescale? For instance, if the first month I operate my business I have a loss of $50,000 (have to buy initial supplies and equipment, hire employees, etc.), but in the second month sales start taking off and I net $20,000 do I (a) have a total loss of $30,000 and pay no tax or (b) owe taxes on the $20,000 in month two?
This can be extended, e.g. profit and loss can be calculated on a weekly, daily, or even hourly basis. In the extreme case, this is no longer a tax on profit but on revenue (which essentially runs any business that has a smaller margin than the tax rate out of business since every dollar of revenue coming in results in more tax liability than the business actually nets).
Before you say "obviously profit and loss should be calculated in a yearly cadence" I would note that the choice of a year is fairly arbitrary and this taxation scheme would greatly disincentive any sort of capital allocations that would take more than a year to payoff (as a small example, would incentive leasing equipment annually vs. buying outright).