Stocks are also IOUs on a certain level but they are not far removed from a real productive assets creating real value (the company that the stock gives you ownership of). Things can get speculative with stocks too, though usually far less than in the cryptocoin realm.

If you add up all the IOUs in the economy, financial assets are cancelled by financial liabilities and what remains is real stuff which in economics is called "investment". The nomenclature can be a bit confusing because in finance they use the same word to mean the contract, the claim or the IOU not the physical asset.

The equation for GDP is Consumption plus Investment (sometimes government consumption and investment and exports are broken out). Investment is not financial assets or money or bonds which all cancel out at the global level. Investment means factories built, equipment built, less tangible things like knowledge and intellectual property count too in theory but are sometimes hard to measure. It's important to have a good level of investment. Promise hoarding can crowd it out when things get too speculative. The economy can get into a bad Nash equilibrium where people are chasing IOUs instead of producing real value.

You are only permitted to own real stuff to the extent that other people can be somehow dissuaded from taking it, and instead of working against you, can be convinced to work together and to trade instead of taking things by force. It’s all trust or the lack of it, just agreements to keep moving society forward instead of reverting to more competitive models of subsistence