Better yet is to make one's own oat milk; I agree that they're exorbitantly expensive, albeit (here in the US) the dairy industry is immensely subsidized and so we consumers don't see the true cost of our cow's milk.
It's all the same problem. Allowing a cartel of dairy processors to control to production and delivery of dairy products is equivalent to allowing a cartel of producers (in the case of almonds) and big food is the same.
All benefit from extensive subsidy (that corn syrup isn't market-based), with the added risk of food security issues, especially for nut milks that are exclusively sourced from a few counties in California.
Leaving aside the issues with directly comparing plant milk with actual milk, the economics are not what you lay out. Oat milk is really yet another industrial corn product. The caloric content isn't cloudy stuff squeezed into water, it's fructose from corn. In the quest for price stability, corn subsidy is one of the main knobs used by USDA from a subsidy and policy standpoint.
I haven't done a deep analysis of this, but I do buy milk from a farm with a smallish herd and a direct-to-producer model that allows the farmer to actually operate his own dairy and sell milk at a profit. Even with the small scale of the operation, milk costs about $6/gallon. When you buy milk from a scaled dairy operation, the economies of scale are massive... you have 1-2 processing plants handling the milk of tens of thousands of cows.
The plant milk phenomenon is all about allowing massive agribusiness to take over a big product category that has remained regional.
This is just factually untrue. Here's the ingredients for the most popular oat milk brands, found from a Google search:
1. Oatly: Oat base (water, oats 10%), rapeseed oil, calcium carbonate, calcium phosphates, iodised salt, vitamins (D2, riboflavin and B12).
2. Califia: Oatmilk (Water, Oats), Sunflower Oil, Minerals (Dipotassium Phosphate, Calcium Carbonate, Tricalcium Phosphate), Sea Salt
3. Pacific Foods: WATER, OATS, OAT BRAN, CONTAINS 1% OR LESS OF: GELLAN GUM, SEA SALT, TRICALCIUM PHOSPHATE, VITAMIN D2. *ORGANIC
4. Silk Oat Yeah: Oatmilk (Filtered Water, Oat Extract), 2% or less of: Sunflower Oil, Vitamin and Mineral Blend (Calcium Carbonate, Vitamin A Palmitate, Vitamin D2, Riboflavin (B2), Vitamin B12), Malt Extract, Dipotassium Phosphate, Gellan Gum, Sea Salt, Sunflower Lecithin, Locust Bean Gum, Ascorbic Acid (to protect freshness).
None of these have corn syrup or added fructose.
You don't need plant milk for that. For proof just see Fairlife, the Coca-Cola Company's attempt to turn milk into a branded good.
And people tend to trust the marketing and the big text on the label more than anything sometimes. The label says it's healthier, that promo on TV said cool young people consume it, etc. So you won't stop to "objectively" evaluate the product. Many blind tests for products turned over results that were completely the opposite of what the market was saying via sales numbers. Because without the label and the marketing driving the decision people had to fail back to good old fashioned taste (ex: this tastes better) and common sense (ex: this offers more for the same taste / price).
Also, the use case and marketing strategy for your two scenarios are different. A product like Snickers is a quick on-the-go fix of food. The marketing connection for a salad is different -- the money and full feeling is in the dressing. Many people translate "salad" as leaves with goops of fat/sugar on them. If you are a oil/vinegar person, you're an awful customer for Kraft.
The food industry is attacking traditional, lower margin foods to make more $. The best example is the push against milk in favor of nut and grain based white liquid. All of the substitute products are mostly water and corn syrup, but are sold at dairy prices.