The number 7 is seen as lucky in slots and pop culture because it pops up often...

https://www.bookmarkidea.win/the-number-7-is-seen-as-lucky-in-slots-and-pop-culture-because-it-often-appears

The number 7 is seen as lucky in slots and pop culture because it pops up often in classic symbols like 777 on reels, which usually means a jackpot. Its luck ties into things like the seven-day week, making it familiar and special