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

https://damiensniceword.readspirex.com/posts/why-is-8-lucky-in-some-cultures-but-7-is-lucky-in-others

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