I guess it makes some kind of sense from an advertising point of view. Why promote the fact that your product makes a mess when it's supposed to be a cleaner? From that conclusion, we get commercials where we don't even see the product being used. None of the people brushing their teeth in commercials is ever using toothpaste:
If you've ever brushed your teeth with slightly less toothpaste than you normally use, you know that if you continue dragging those dry, lifeless bristles around your mouth, you're imposing a shitty start to the day onto yourself. If you see someone brush without toothpaste at all, they're just going through the motions of life. They have no real reason to live. They're doing the absolute bare minimum needed to survive. Toothpaste foam is a sign of effort, and yet look at all those people in the pictures. Brushing bone with something as dry and flavorless as a sun-bleached bone is the pinnacle of delight for them.