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

https://marcoscooljournals.lowescouponn.com/what-does-777-mean-on-a-slot-machine

The number 7 is seen as lucky in slots and pop culture because it pops up often in classic symbols like 777 on reels, which usually means a jackpot. Its luck ties into things like the seven-day week, making it familiar and special