There is no fully "trusted" source of the rate. We do have rates from the leading stock exchanges, we do have average or weighted aggregates of those rates. But still that sources are "points of weakness" in the system. We can't trust them that way like we trust blockchain information.
No offense, but you're absolutely incorrect. Market prices are the only "real" prices. Please understand the concept of "price discovery".
https://en.wikipedia.org/wiki/Price_discoveryEven Fiat like USD, has no "official" price in relation to other currencies. Various exchanges and
banks trade currencies and rates are always floating. Rates tend to converge on a single price at any given time
due to arbitrage. This is true whether its dollars or Bitcoins.