Why would a completely free market economy not be a practical one for any major country?

A completely free market would mean that all the services would be provided only according to supply and demand, and this has some drawbacks:

For example, some parts of the economy should have different standards than prize, and these are education, infrastructure and health. In a completely free market, the buyers would for example be tempted to buy the cheapest bridge, but the cheapest bridge might be actually too dangerous to drive on it.