It might be helpful to read about a simpler Proof-of-Stake system like the one ETH is working on before trying to grok Steem. Steem is really just Proof-of-Stake with a game built into it.
But upvotes on Steemit don't take currency from person X and give it to person Y
They do though. There's a third currency in play, rshares, that no one likes to tell you about. It's the one the voting system runs on. You get it from staking in the form of voting power, but your actions with it are restricted; all you can do with it is vote, decline to vote, or flag. When you vote the rshares are transferred to the person you voted on, and when post payouts happen they're forcibly converted to Steem and SBD.
So I'm having trouble understanding how "real" money gets injected into STEEM
People invest it in order to get their share of the staking rewards, and then either vote them or convert them to cash. Why does anyone buy a bond? I wouldn't necessarily argue if you thought the Steem blockchain deserved a junk rating but people buy those all the time too.