The number 7 is seen as lucky because of its long-standing cultural...

https://www.easybookmarkings.win/the-number-7-is-seen-as-lucky-in-slots-and-pop-culture-partly-because-it

The number 7 is seen as lucky because of its long-standing cultural significance and its use in slot machines, especially with the classic 777 symbol which often signals a jackpot