The number 7 is seen as lucky in slots and pop culture because it’s been linked...

https://wiki-spirit.win/index.php/Why_Does_Seven_Show_Up_in_Calendars,_Religion,_and_Games_at_the_Same_Time%3F

The number 7 is seen as lucky in slots and pop culture because it’s been linked to good fortune for centuries. Classic slot machines often feature triple sevens (777) for big wins