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

https://zanesnewthoughtss.evergrovio.com/posts/what-does-bar-mean-on-a-slot-machine

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