The number 7 is seen as lucky in slots and pop culture because it's tied to...

https://wiki-club.win/index.php/What_Do_Bells_Mean_on_Classic_Slot_Machines%3F

The number 7 is seen as lucky in slots and pop culture because it's tied to classic symbols like the 777 jackpot combination on reels. It also relates to the seven-day week, a familiar cycle