Steem blockchain produce constant amount of STEEM tokens everyday (1 STEEM per block every 3s).
Those tokens are distributed among all users according to votes of all users.
We all know, that it is not so easy to write a post/comment which will receive tons of upvotes.
So if you will spend 3 hours writing awesome blogpost, and after a week (voting period) you will endup having 3 STEEM tokens... question is, would you give away those tokens for free? Probably not for free...
Also, there are people on the platform, which like to gather STEEM tokens. Why? Because if you have more STEEM tokens you have greater influence on the platform, your posts will have greater visibility, and you have more to say about who will receive more tokens next time.
Price of STEEM tokens are determined by free market on bitcoin and concurrencies exchanges: https://coinmarketcap.com/currencies/steem/#markets
On exchanges people which do not care about influence on Steem sell tokens to people, which want to have influence on the platform.
It is a free market in purest form.
Don't believe? You don't have to. There is no need for trust on Steem. Eferything is open source: github.com/steemit/
Everything is transparent (blockchain explorers): - https://steemdb.com/ - https://steemd.com/ - https://steemstats.com/
Where is the money coming from, other than speculation?
It barely even hides this fact, given that all its content is about how to generate more content and keep all the wheels turning.