Why are the public sector and private sector "imperfect institutions"?

The view of a benevolent government that responds with precision and efficiency to its citizens' wants isn't accurate. The market system of the private sector is far from perfectly efficient, and government's economic function is mainly to correct that system's shortcomings. But the public sector is subject to deficiencies in fulfilling its economic function. "The relevant comparison is not between perfect markets and imperfect governments, nor between faulty markets and all-knowing, rational, benevolent governments, but between inevitably imperfect institutions." Because markets and governments are both imperfect, it is sometimes difficult to determine whether a particular activity can be performed more successfully in the private sector or in the public sector.