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

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 often appears as a winning symbol, like the classic 777 on slot machines. Its luck may also come from the seven-day week, which is a familiar, repeating cycle