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

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 shows up often in old-school reels, like the classic 777 jackpot on machines. It also feels familiar and positive since we have a seven-day week