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.
The fact that majority have concluded that gambling is more of losses than wins, then I don’t have to doubt it anymore. If we don’t have personal experiences on gambling, then at least we should learn from other people’s experiences, that way we won’t be misled in gambling, that would help us to prevent from bigger losses when we gamble at a consistent basis.