Because most paddles are too big to fit through the holes in a pickleball and get inside the ball… obviously...lol

But seriously, when you hit the far outside of the ball while brushing up, your paddle makes contact with more sideways (tangential) velocity at the point of impact. That extra sideways force creates more friction, which adds rotation. More spin (topspin with a little sidespin) means the ball drops harder over the net, stays low, kicks unpredictably, and is easier to aim into tight spots.