To be fair, cow's milk is also about 90% water. You're right, though, that the ingredient cost using commodity oats and rapeseed oil should only be about 4-5 cents per liter. The price of cow's milk, on the other hand, is a lot closer to the price of the inputs (corn, alfalfa, grazing land, etc.) which are about 16 cents per liter.

I'm hopeful that eventually the price of oat milk will approach the cost of its inputs, but it might take a while.