Why a (fucking) bet? At its core, She’s All that is an unlikely couple trope. We wouldn’t expect the weird, artsy kid to date the high school quarterback. And Janie’s and Zach’s personalities and priorities are different enough that it’s believable that they would never meet or get to know each other under normal circumstance.
But there are other ways to throw two characters together. Janie could have been asked by her art teacher to help Zach (who was probably President of the Student Council or something) come up with a theme for Prom. Or Zach could have needed a job at the Falafel Hut, for some reason. It’s believable that their personalities would clash some, but, as in the actual movie, they find it easy enough to find common ground once they get to know each other.
So, why a bet? Aside from it being a way to get the characters together, it’s also provides motivation for Zach to try to breech Laney’s emotional defenses, instead of backing off forever, which most teenagers would do when faced with Janey’s steely gazed world weariness.