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

https://casinocrowd.com/why-do-people-say-seventh-heaven-unpacking-the-mystery-behind-the-phrase/

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