The number 7 is seen as lucky in slots and pop culture because it's tied to...

https://meaninngs.com/what-does-the-number-7-mean-in-casino-games-and-popular-culture/

The number 7 is seen as lucky in slots and pop culture because it's tied to classic symbols like the 777 jackpot combination on reels. It also relates to the seven-day week, a familiar cycle