You're already right that it's just a mental thing, luck doesn't have anything to do with someone losing or winning in gambling, it's always a matter of maths and probabilities but then again, it just so happens that luck is such a good scapegoat that it's been that way for how long we've got this concept, luck is our refuge for when things aren't explainable or the human mind can't comprehend, I guess we just have to go out of our way and say that it is what it is when it comes to luck and if it helps people get comfortable with gambling not overthink or stress out on such things, it's probably for the better that we just leave it as it is.