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

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 symbolizes good fortune and completeness, like the seven-day week. In classic slot machines, hitting triple 7s (777) often means a big win