The number 7 is seen as lucky in slots and pop culture because it often appears...

https://wiki-saloon.win/index.php/Why_Is_the_Number_7_Lucky_in_Slot_Machines%3F

The number 7 is seen as lucky in slots and pop culture because it often appears as a winning symbol, like the classic 777 on slot machines. Its luck may also come from the seven-day week, which is a familiar, repeating cycle