Because there’s plenty to win on free Triple Lucky 7s by Imaginative Gaming. And while there’s nothing representing that here, it’s never good to turn down luck.
It’s also considered pretty lucky in Chinese culture. The Magnificent Seven, Seven Samurai, The Seventh Seal and The Seven Year Itch. Of all films involving a number, 7 has to be up there. It’s the number of dwarves, deadly sins, days in a week, wonders of the world, continents and colours in the rainbow. There’s something pretty awesome about the number 7.