'Cause on a traditional arrow shaped paper plane, the folded wingtips act like vertical stabilizers and add longitudinal stability. Remember, they're on the very back of an extreme delta wing. If you were a model airplane hotdog (like I was), you could trim out your paper plane for longest flight or a circular path by bending the trailing edges slightly for yaw control.

Rutan was the innovator of controlling wingtip vortices and converting that wasted force into lift. Not much of a tip vortex on a paper plane.

BTW, the tip vortex is from high pressure air under the wing spilling out toward the top of the wing, not vice versa. I flew into a tip vortex coming from a C-130 while on final approach to Huntsville AL. It flipped my Cheetah completely inverted in a right roll in less than a second but I instinctively added full right aileron to complete the roll and landed uneventfully. Took 10 minutes to get my underwear out of my tush.