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

https://garrettjagb275.yousher.com/what-are-seven-league-boots-supposed-to-mean

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