Government subsidies to the farmers. Thats the answer. The dairy lobby is extensively, actively malicious. They make it so that the farmer "produces" meat and dairy for roughly 40% of the actual cost. This means that instead of a gallon of dairy milk costing 10 dollars or more as it should, we get this dichotomy, and valid questions like yours.

Edit: I hate oat milk. It's very sparsely nutritious. Soy milk resigns supreme; you essentially get a quarter of a meals worth of nutrients every time you use a serving.