點解明知long run profit=0都要做生意既?

Cost includes payment to all production factors, e.g. rent, wages, interest on capital, depreciation, etc. Cost also includes payment to entrepreneur. The owner's revenue is just sufficient to cover his costs of undertaking the business, which include payment for managing the business and the cost of risk bearing.

The owner is also a factor of production. There being no profit is not the same as no income for the owner.