Competition in some industries is simply not profitable. So, for example, the construction of the metro costs insane money and expensive technologies, and none of the private entrepreneurs wants to take on this business, and even if they do, they have no opportunity. And the metro is necessary for a modern city, so a state monopoly arises, permitted and enshrined in antimonopoly legislation. For example, a monopoly is allowed in the defense sector of industry, this makes it easier for the state to control product quality.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.