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

https://tiny-wiki.win/index.php/Is_It_Really_Seven_Years_of_Bad_Luck_if_You_Break_a_Mirror%3F

The number 7 is seen as lucky in slots and pop culture because it appears often in classic symbols like 777 on slot reels. It’s also tied to the seven-day week, which feels familiar and steady