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

https://wiki-spirit.win/index.php/Why_Does_Seven_Show_Up_in_Calendars,_Religion,_and_Games_at_the_Same_Time%3F

The number 7 is seen as lucky in slots and pop culture because it often represents good fortune and completeness. For example, classic slot machines show triple 7s (777) as a winning combo