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

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 represents good fortune and completeness, like the seven-day week. Classic slot machines feature the 777 combo, which is a top prize symbol