Because corporate taxes are paid on profit (revenue-expenses). So as a company I can claim a lot of stuff as a expenses. Executive salaries? Anything left after that? Lemme quickly create a separate company in some low tax jurisdiction and pay them for marketing expenses, and suddenly I have no profits, and nothing to tax. Regular people can't do that.
You also generally can't tax the revenue instead of profits (except perhaps few percent of revenue as the minimal tax), because different companies have hugely different amounts of revenue and expenses.
