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

https://atomic-wiki.win/index.php/Why_Do_Designers_Like_the_Number_7_as_a_Symbol%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