Why is lactose never mentioned as a fuel for cycling, either for short fast races or endurance? Sucrose, dextrose, glucose and fructose, but never lactose. Why not, what's wrong with it as a source of fuel?
Glucose (dextrose is the same thing) has a special role in metabolism - as well as being a nutrient sugar in its own right it's a vital intermediate in breaking down more complex sugars, starch, and glycogen, our bodies' main store of carbs.  This all means we're able to get energy from it very efficiently (excepting cases of diabetes).  This is why energy drinks (the ones with calories in, not just caffeine and flavouring) use it.
Maltodextrin, used in energy gels and some energy drinks, is pretty much just a string of glucose molecules.
Fructose is metabolised rather differently, but the pathway is connected to glucose metabolism, so it's not exactly parallel.
Sucrose is a glucose and a fructose joined together, so also a very available source of energy.
Lactose is broken down by lactase (in those individuals that produce it) into glucose and galactose.  The latter in turn then ends up as glucose by a variety of routes, probably slower release than glucose, but faster than starch. It would appear to offer no advantage as a parallel source of energy to glucose, while not being any cheaper and intolerable to some people.
So if you like it, you can certainly get energy from it, and milk can often be bought easily - on a recent warm ride it would have been the only way for me to get a chilled non-alcoholic drink in one shop (but I don't like it as a drink so I had an ice cream and water instead).  For a manufacturer trying to make money, it would make their products more expensive for no sales benefit.