The answer to your question is simple, and it happens to be the fact that the bitcoin and other cryptocurrencies are decentralized,now what does that mean?, it simply means there is no body,rules,organization,government nor Central Bank that acts as an overseer over this network/currency
This network/system/currency Is controlled only by the Individuals,users,enthusiast,investors etc in the system,only them can affect and influence the price/value in many ways, all depends on the demand/supply of the particular cryptocurrency coin
Exactly, and that's why its impossible to know the exact circulation of bitcoin or any other coins since no one knows how many bitcoins have been lost due to failure of PCs, crashing of hard drives having bitcoin wallet or private keys. Nobody rules these decentralized currencies and that's why no one can help you when your coins are lost of stolen.