Contrary to popular belief, kibble doesn't actually clean the teeth. For the most part, dogs (and some cats) actually swallow their kibble whole and don't chew their food at all. You'll know this if you've ever had the misfortune of seeing your pet regurgitate his kibble, when those pellets come back up the same way they went in! This means the pellets don't have a chance to come into much contact with the teeth, and even if they were to be crunched, they aren't really hard enough to have an abrasive action that's effective enough to scrape tartar from the teeth. If kibble is chewed (which may happen in some smaller dogs and cats) the sugars and refined carbohydrates are actually more likely to get impacted into the gum line, which is detrimental for the long term health of the teeth and mouth.