Because then it loses value.
Think of it as if a publicly traded company just issued bajillions of shares onto the market. This is what dilution is, and the price of each individual share plummets, thus screwing over all existing shareholders. Normally, dilution is limited, but with this, i would be like issuing many times more shares than already exist on the market.
If you have more gold than everyone on earth and you flood the market people can use it for any numerous uses that help humanity instead of making people rich.
Science research is often hurt by the fact that materials like gold and diamonds have such high prices.
More platinum group metals and gold might not make a big difference on Earth.
On the other hand decadbonization requires metals like nickel and cobalt as well as methods for producing steel that don’t release CO2 in the atmosphere. Trouble is it is still hard to be terrestrial sources.