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

https://charlie-wiki.win/index.php/What_Does_777_Mean_on_a_Slot_Machine%3F

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