Single bets are easier to win than multy bets (parlay) because you only need to make 1 correct guess to win the bet. While multi bet (parlay) is much more difficult because you have to guess everything right to win the bet and that is the reason why single bet is better than multi bet (parlay) based on probability. Both have different risks in terms of probabilities, but multy bet (parlay) can make you win more money if all your guesses are correct.
But if you ask me then I prefer single bet over parlay because I can't win a parlay for long for 4 different match and league every week.
You are absolutely right. How many times I have not made parlay bets I have never been able to win. After that I prefer to use single bets only. Although an acquaintance of mine still makes parlay bets for small amounts and sometimes wins. I prefer to minimize the risks, so I only choose single bets.