Let's say you run a small software company, which is a high margin business. You have $1,050,000 in revenue this year, and $500,000 in expenses, so you make $550,000.
You should be taxed on the $50,000 and the $550,000, not on the $1,050,000. Otherwise, if business expenses were not deductible, low margin and capital intensive businesses would be punished severely. We would have decreased investment in the economy and everyone would suffer for it.
If individuals could deduct their expenses, it would encourage people to spend every penny they make. The mortgage interest deduction is one example where this nudge becomes apparent (albeit real estate has merit as an investment, not just consumption). We already have a low enough savings rate as it is.