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

https://ellassmartperspectives.urbanvellum.com/posts/is-seven-always-about-luck-or-can-it-mean-something-else

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