The money is created by steemit platform. The steemit platform distribute, everyday, an equal amount of currency. This currency(steem) is worth what people are ready to pay for it, and you can see in the past few days, as more people learn and want to join the platform, this currency gains value.

So more people joining the platform create buying pressure for steem (because they want to buy steem power) which makes steem more valuable, which in turn increase the profit makes by people contributing to the platform. It is a self-reinforcing loop.

Hope that make sense :)