... it always starts off nice and smooth and no play in throttle
That may be the key right there. Properly adjusted, there should be a little play. Not much, but some. With the throttle sitting on the idle stop, you should be able to turn the throttle a bit back and forth. If you are continually adjusting the cables so that there's no play, you may be tightening them to the point that there is continuous tension between the two cables. And if so, that continuous tension may be causing the cables to stretch. The two cables should not be applying tension to each other. But that may not be apparent, because they could be in tension, but the throttle system will still work smoothly if it's well lubricated. In fact it might even feel "better" to you that way because it's nice and tight with no "slop"
Try this: Read Kawasaki's instructions on adjusting the throttle cables. I think that it's in the owner's manual, but I'm not 100 percent sure as I'm away from home at the moment. For sure it's in the Kawasaki service manual, and also in Clymers.
Anyway, when you install your new set of cables, read that procedure, and pay attention to the minimum play specified in the throttle. Adjust your cables, exactly the way Kawasaki says to adjust them. And any time down the road, if you think they need adjusting, take out the manual, read it and adjust them as per the instructions
You may find that you'll stop needing to replace your cables every year.