Free markets don't necessarily lead to the best outcome, and certainly not to the best outcome for a society.
If you go read the actual invisible hand quote from Adam Smith, that so many people are fond of quoting, he is pointing out that the free-market leads to unintended outcomes and side-effects:
"By preferring the support of domestic to that of foreign industry, he intends only his own security; and by directing that industry in such a manner as its produce may be of the greatest value, he intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention."
His argument is basically that sometimes merchants accidentally do things that benefit society. The unstated opposite is the issue.
The concept of free-markets is nebulous, and thus more of a political football than a serious thing. Every culture has limits on what it will allow of the market, and what is considered not allowed. A monopoly that harms the consumer is not allowed, but we do allow monopolies in circumstances where it would be massively inefficient not to operate as a monopoly (utilities is the most common one, e.g. only one company is allowed to own power lines in your neighborhood).