When you start to explain it like the company has a checkbook, obviously that approach will fail to explain the source.
In essence Steem just provides users with worthless virtual tokens, not money. The tokens only acquire value through the markets, because some investors are paying good money to buy them. Thus the users of the platform can cash out these worthless virtual tokens with bitcoins (which is another worthless virtual token that acquires value through investors) and then to cash.
So you have this type of answer:
-Where does the money comes from?
-There is no money. You are paid in tokens. These tokens you can then sell at the market for bitcoins and/or currency.
Or
-Where does the money come from?
-Investors (indirectly)