Crypto currency itself is not by default decentralized the reason for such misconception is because the term is always common among crypto currency and any currency that would want to claim its rightful place had to be decentralized in nature or claim to be one. There is really no defined line between centralization and decentralization. Today even though bitcoin claims to be a decentralized crypto currency, we have centralized exchange sites, we have centralized wallet services where you can exchange for any coin or sell directly.

And most importantly, despite what we are made to believe that mining is decentralized just like what @bitmover illustrated, the moment any of the major miners merge, they can conveniently control the direction of price and nothing anyone would be able to do about it because there wont be any regulator to forestall such move just like the case it would have been in the business world where merging of two firms is likely to cause any form of monopoly of the market they operate.