Single bets are generally better than multi-bets basically because you only have to win one in order to win your bet. Whereas, in a parlay, you have to win 2 or more bets to win your bet. That alone explains why single bets are better. The odds further show you that. Parlay gives you considerably higher odds because it's harder to win. The chances are low, risks are high, therefore, rewards are also big.
However, betting on single bets doesn't mean you should be profitable. It's hard to make money in gambling. It's just that it's a lot harder in parlay betting. Placing single bets doesn't guarantee profit. On the contrary, you're even most likely to lose in the long run. It's just that in multi-betting, it's much more possible your bankroll is drained of its last Sat without winning any.
But essentially, the money we deposit into gambling still carries the same high chance of losing, but some people may be lucky enough to achieve profits equal to or greater than their initial investment. Some people may prefer parlays, and even if parlays aren't widely popular, that's understandable because the odds and risks are vastly different, so only the brave enough to try them are willing to do so.
The difference you mentioned about single and double bets is true. The chances of losing are the same, but the way you lose them depends on the speed. With single bets, it might take a while or be a series of bets, while with double bets, you can lose everything instantly.