The number 7 is seen as lucky because it appears often in slot machines, like...

https://oscar-wiki.win/index.php/Is_777_a_Religious_Thing_or_Just_a_Casino_Thing%3F

The number 7 is seen as lucky because it appears often in slot machines, like the classic 777 jackpot combo. It also connects to everyday cycles, like the seven-day week, giving it a sense of completeness