I think when people ask:
Where does the money come from?
they usually mean this:
Who is gonna be on the other side of the trade, when I cash out and sell my Steem Dollars?
And I'd suggest the shortest answer to be something like this:
Behind the Steem Dollars (that you get for posting & commenting) are actual shares in the Steem system. So when you cash out you receive "real" cash for your Steem Dollars, but at the same time you give up some of the shares in the Steem system. And there are people who would like to buy those shares from you.
Why would they want to do buy those shares?
The shortest answer is that they believe (or speculate on the fact) that Steem (with its Steem Dollar) will one day become a ubiquitous momentary system and there will be millions of people buying and selling stuff for Steem Dollars (which effectively is nothing else than "repackaged" Steem) and, as a result, there will be demand for it as a currency just to sit in people's wallets. It's the same vision Bitcoin (or any other crypto-currency) investors have. It's a speculative bet.
So the answer really boils down to this: on the other side of the trade are long term speculators/investors who see in Steem a ubiquitous monetary system of the future.