Gambling is based on luck which is why majority of gamblers are losing because luck comes by chance. Only few gamblers are able to gamble on the right time luck was smiling at them. It's not that all gamblers ain't lucky but are you gambling on your lucky moment in order for your luck to favor you? However, nobody knows when is the right time to gamble in order for you be lucky.
Luck is random in gambling and like you said, no body can determine when they will always be lucky, if that was so possible for a lot of gamblers, that means we should have had so many millionaires in gambling. It's not a hidden fact that gambling leads to more losses than the profit that it can return. So, regarding what OP said, people don't need to personally experience it before they believe, others who refuses to gamble because of the risk must have also seen the harm it caused some persons.
If luck should be predicted, I believe every gamblers wouldn't be losing their bets. The fact that it all depends on luck for a gamblers to win bet, their strategies won't determine their winning. I still believed that at first many gamblers understand the risks in gambling but after some few months of winning or getting more losses they become addicts to it.