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

https://varimail.com/articles/what-do-bells-mean-on-classic-slot-machines/

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