First, I dont want this to be true but it is.
Currency is a unit.
It is a unit in the system of measurement of value. Just like an inch is a unit in the system of measurement for length.
Units (and there have been thousands of different ones) all have 2 things in common.
1- They are not real. They are all made up, everyone of them is just an opinion. Inch, pound, meter, cat 5, g force,currency --- not real.
the are all just an opinion that we chose to share. We all share the opinion that an inch is so long, if we all shared the opinion that an inch was a foot then it would be.
2- All units are equal to a constant.
This is provablely false. Most units of measurement are, in fact, arbitrarily chosen standards of agreement. Currencies are, truthfully, a unit of measurement; but only of value. Value is, itself, highly subjective. No currency has a constant value relative to anything other than itself, and often not even to itself over time. Yes, Bitcoin is both a currency and a commodity, in many respects; it's also a service, and that service also contributes value to the users.
This. ^^^
It seems the OP has common misconception that value is objective, intrinsic to the thing being valued and fixed on an absolute, rather than a relative scale. A unit of measure is only ever an approximation of relative measurement, and not a perfect quantity of fixed objective existence.
Much can be gained from realizing that value is subjective and relative. OP also needs to consider that there is no logical inconsistency in the concept of 'relative value'. Just because it cant be resolved to an objective and fixed unit of measure, does not mean it cant be a unit of measure. Value is assigned by cognative projection in the first instance, and then by social consensus as the process of invoking the value vested in any common medium, that might be called currency.
Currency btw, is also a relative term. Something gains 'currency' as it gains acceptance as medium of exchange. It is essential fwiw, that an honest currency must allow the value to fluctuate as demand and supply dictate. The requirement to do otherwise, is not sound economics, but rather corruption.