It's very simple.Although individual bitcoins enter the Bitcoin economy as miners are rewarded for processing transactions, it's much more helpful to think of all 21 million bitcoins as having been created when Satoshi Nakamoto defined the Bitcoin protocol and launched the Bitcoin network in 2009.

The reason for this is that the Bitcoin protocol specifically defines and controls when and how a limited total number of coins are rewarded to miners for the job of securing the Bitcoin network. These "bitcoins" are really just mathematical tokens which are very carefully controlled by the network protocol to prevent counterfeiting, theft, etc.