The first response that comes to my mind is "because if the butterfly were trying that hard to escape the kid, it would fly above the kid's reach, and the kid would give up." When I look at the scene, I see a kid chasing a butterfly, and a butterfly too stupid to realize it should flee instead of simply dodging.

Animals on the intelligence levels of butterflies (which, keep in mind, have specific mating flight patterns they use to tell other members of their species apart from things like ribbons and stray flower petals,) don't seem to even have retreat instincts, just avoidance instincts. They can't recognize persistent pursuit. A fly won't hesitate to land on a person who has been trying to swat it for minutes on end.