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.
Gambling depends on some portion so if someone tells me that in gambling he will gamble entirely on luck then I will say that his assumption is wrong and if he gambles with this assumption then he will suffer financially from gambling. If everyone was successful in gambling depending on luck or if luck could make a gambler win, then every person in the world would gamble and every person would be successful in gambling. You can notice a thing you know about a thing but you can tell about that thing but you can't tell about the thing you know nothing about no matter how simple the thing is. Now when it comes to gambling if we don't have minimum understanding of gambling and we gamble only depending on luck then that is a big problem for us. We need to know about this first as well as try to gamble with our own strategy only then we can expect to win.