To make this assumption, one must assume people are always rational. They are not.
I'm not sure I've ever understood this argument. It doesn't matter if people are rational. If businesses make the wrong decisions they go out of business, and there are consequences for individual people based on their wrong decisions.
You said that the market
always makes a
better decision than the government. I suppose it depends by how you define "better" but it is not always the most efficient. For the market to attain the most efficient price for everything, all people must have all knowledge needed to make a rational choice and to never produce a less efficient decision. We know that individuals and crowds make irrational decisions that can have tremendous consequences that do not need to happen.
For example, the Great Depression became the Great Depression beginning in about 1930 when the first wave of bank failures hit. There were several more until 1933. There is empirical research concluding that banks that failed in the 1930s were generally as strong or stronger than banks that the did not fail. This is not rational. If people were always rational and all information was available, then people generally would know which banks were the strongest and put their money there instead of weaker banks. But this did not happen as people became fearful and panicky, and pulled their money out indiscriminately, which had enormously damaging consequences to the economy.
People have to always be rational all the time for markets to always work. Most people are rational most of the time, thus markets mostly work. But when people become fearful and panicky, or conversely, greedy and euphoric, the outcome of their decisions can be catastrophic to the economy in general.