Who said betting on a single match is reliable? As we say that the money you deposited in a casino is highly likely to be lost, so is the money you deposited on sport betting sites, regardless of the odds that you go for. Only what people can say is that parlay is riskier, but it has its own advantages.
You are right that every dollar deposited into a casino is lost and should be considered lost until there is a high chance of winning or it has been won, but some gamblers don't consider it that way; they believe that every game with low odds presents a higher chance of winning than placing a parlay bet, just as we all know that the higher the number of games piped up together, the higher the chance of losing. That's how some people also consider single bets.