Buying a security is like betting in a casino. The house has the odds, but they occasionally allow someone to win in order to keep the suckers at the tables.
It's not your brokerage firm, because they don't control stock prices. It's certainly not the companies who issue the stock, because their senior officers cash in their options when their stock performs well. It's not the institutional trading desks, because they are competing with each other to make a profit.
It must be the proverbial "they" or "them" that no one seems to know.