Why do people steal Nesquik in grocery stores?

I work at a grocery store as a grocery stocker and always find half empty bottle of Nesquik. Customers just drink half the milk without paying for them and leave. They do the same thing for the the big bottles! I understand why i find empty cans of whip cream. Because people huff them at night but stealing chocolate milk especially the quarts is crazy. How do you drink a half a quart in seconds???