It is because people have become so accustomed to the existence of fiat in general, and to the monetary system in its current form in particular, that it is now almost impossible for them to understand how to measure the value of things such as bitcoin without referring to the existing means of measurement coercively imposed by governments or by influential bankers. The answer, however, is quite simple: if we were suddenly deprived of the ability to transact in fiat, humanity would return back to a barter system, in which one type of good is being measured in terms of all other goods simultaneously and which is, therefore, rather very inefficient. But what is interesting about such a system of economic relationships is that it eventually creates the most saleable good - money. But given that people already know that bitcoin can be, and, in fact, already is money, the return to a barter system is highly unlikely. In that case, bitcoin would, from the beginning, be used as a means of exchange, and all the goods and services in existence would measure only in its terms. So, how would we calculate the value of bitcoin if fiat suddenly disappeared? If you can buy five cars with your bitcoin, then the price of bitcoin is five cars.