It's because the currency gold is priced in is going down year by year. Why should you spend your gold if your fiat is worth less tomorrow?
That's besides the point that gold is practically unusable as a currency for convenience reasons (divisibility, verification, storage, transportation).
A great currency is a currency that preserves your value over time, as currency is what you earn for spending your time and skill. You shouldn't be forced to take a risk to maintain your wealth. Today it's risky not to divest your fiat.
