I believe this is due to the fact the price of Bitcoin fluctuates, sometimes quite rapidly. Also, the fees are to allow Coinbase as a company to operate - no income means no Coinbase.
On the other hand, if you hold any alternative cryptocurrencies you could achieve a far more accurate price.
Mindgeek, still interested in AoE? Add me on Steam then ;) Just see your post on my announcement!
True... good point.