He has to give hitters a reason to swing at a curve.
Right now, there isn’t one.
Hitters have less than 1/2 of a second to determine what a pitch is, if it is going to be a strike and if the should swing. If Kimbrel is tunneling his pitches hitters will have a lot harder time determining all of that and somebody would chase at a bad curve ball in the dirt at least once. If the curve is coming at a different angle they can just ignore it and wait for a fastball.