They aren't running a business. Running a business and running a government are two different things.
You understand the point that people who have successfully run a business in one sector wouldn't necessarily be successful or even mediocre in another sector. There's successful business men who have ended up complete failures in the same sector because they were reliant on one model and lacked the flexibility.
Do you believe Simon Jordan, Alan Sugar or Peter Swales would have made good politicians?
Is it possible that people who have experience of working in a union, local government, representing constituents and serving in the shadow cabinet have better understanding of the mechanisms of government and are better qualified to work in government than a quantity surveyor and landlord?