Not only are losses not taxed, but they are often
deductible against other taxable (profit) income. I believe this is mostly out of a sense of fairness, and also because profitable business activities often start out as unprofitable, and we don't want to discourage that start-up phase, or we might not ever get profitable businesses.
edit: losses are not deductible in perpetuity, however. For small businesses, after several years of losses, the business is presumed to be a not-for-profit ("hobby") activity for which losses are not deductible. And larger businesses can only continue to generate losses by investors dumping more money in only to lose it. (U.S tax system)
>We want to incentivize profits
There is already plenty of incentive without any government intervention.
And as others have pointed out, taxes are heavily based on a ability to pay. For example, while cancelation of debt (forgiven loans) result in taxable income (U.S), there are exceptions for people who are insolvent (and therefore obviously can't really pay the tax).