You're focusing on a game with player vs. house odds, like a casino. Online betting platforms do offer some of these games but they are clearing markets for gambling; they manipualte the odds to arbitrage wagering and take a cut regardless of the outcome. It's all about volume. If you make a huge wager on a long-odds parlay, they no longer look the same for the next (or other side) of that specific (or components of the) wager.