We know no such thing. The only objective fact is that there have been fewer financial crises since the dollar went off the gold standard than before.
Financial controls don't work perfectly, but having money tied to a commodity is fraught with its own difficulties. It's actually quite handy to have fine-grained control over the money supply that is not coupled to external physical circumstances because then at least we have the option of controlling it to produce better outcomes even if we don't always succeed.
Following that logic, every country can simply print fiat money until everyone is rich.
This idea of commodity money is far detached from how it actually works. Thread has several good links if you want to learn how it actually works.
What is the reason behind so many people believing this? Do they teach it like this at school? (This resurfaces everytime money is discussed here)
Consider the 2007-2008 financial crisis. Abstractly, investors believed that homes had significant value until, one day, society realized the value didn't exist in reality but only "on paper." We know that investment bankers were hiding the problem until the proverbial lid blew.
You might consider that our current financial paradigm has never been tried before in human history. For thousands of years, currency was based upon various measures of value which attempted to be as objective as possible. Semi-rare materials (e.g. gold and silver) and difficult-to-replicate coinage were some examples. The US dollar was decoupled from the gold standard under the Nixon administration. The end result is, one dollar now holds value relative to the amount of dollars in existence. To increase the relative amount of precious metals into the world is relatively difficult (actual physical labor is required), but to increase the total amount of dollars in existence is as simple as updating a database entry.
If financial controls work exceedingly well, the current economic system is viable. We know, however, that they don't.