Let's say you operate a low margin business, such as a gas station. All numbers are before tax.
For the year, you spend $1,000,000 paying for fuel, convenience store supplies, electricity, wages, etc. You get $1,050,000 in revenue from your customers, for a fairly realistic margin of 5%.
Now let's say you run a software business. You pay $500,000 in electric bills, computers, wages, etc, and collect $1,000,000 in revenue for a margin of 50%.
Would you rather live in a society where the gas station pays MORE tax than the software business? This would be the case if you taxed revenue instead of profit. If the revenue-tax rate were 4%, the owner of the gas station would be left with $10,000 at the end of the year, and the owner of the software business would be left with $460,000. Bringing these numbers back into normal, profit-based terms, the gas station owner would be paying an effective tax rate of 80%, with the software business owner paying an effective tax rate of 8%.
If businesses were taxed on revenue instead of profit, low margin businesses and capital intensive businesses would be punished disproportionately. Businesses often face unplanned expenses or loss of revenue. This would have the effect of forcing businesses to raise prices to compensate for the increased risk, and would force lots out of business. This reduced economic activity would be bad for consumers, producers, the government, and society as a whole.
Individual people's income taxes cannot be treated this way, because the closest analogy to "business expenses" for a person's lifestyle is their cost of living. If cost of living were deductible it would be highly gameable and ripe for abuse by numerous parties.