Most numerology terms have at least a rough operational definition — "leadership," "adaptability," "compassion" all describe something you could, in principle, observe in a person's behavior. "Lucky" doesn't work that way. It describes an outcome, not a trait, which makes it a strange thing for a personality-reading system to promise.
What numerology sources actually do is borrow luck from elsewhere. 7's reputation as lucky predates the personality-numerology tradition by millennia, showing up in religious texts, folklore, and games of chance across many cultures. 3 gets folded in more from numerology's own reading — associated with joy, expression, and social ease, traits people tend to describe as "things going well" even when they're not literally about fortune.
Numbers like 8 also get called lucky in specific cultural contexts — most notably in Chinese numerology, where 8's pronunciation echoes the word for prosperity — but that's a separate tradition from Western destiny-number calculation, not a claim within it.