Your questions offers a clever contradiction. The normal answer is that people are lucky by random chance. But there is no “why” to explain random luck.
In real life, meaningful luck is more like what Seneca described, “Luck is what happens when preparation meets opportunity."
Sun Tzu goes a step further. He suggests that the best strategy incorporates as systematic search for opportunities that we can take advantage of. His view was more like Samuel Goldwyn’s, "The harder I work, the luckier I get."