The number 7 is seen as lucky because it appears often in slot machines, like...

https://meaninngs.com/what-does-the-number-7-mean-in-casino-games-and-popular-culture/

The number 7 is seen as lucky because it appears often in slot machines, like the classic 777 jackpot combo. It also connects to everyday cycles, like the seven-day week, giving it a sense of completeness