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.
No type of gambling is reliable, you take risks whenever you put your money on the line to win more money, although some types of games might seem easier to win than others. When you compare single bets and parleys logically the single bets will seem easier because it is only one game unlike parleys that are more than one game. But I think that it is a matter of preference, despite that parleys are more than one game some gamblers still prefer it. Single games are faster and that is why many bettors goes for it, it is as simple as that.