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More to the relevant point, those alt milks are still cheaper to produce and Starbucks has the scale to do so. You know what it takes to make oatmilk? Oats, sugar, water, small amount of oil. Almond milk? Replace oat with almond, except you can use more of the material.
In oatmilk the sugar and oil are optional ingredients for taste and texture. Almost all oatmilk brands contain salt for taste.
I’ve only found one brand that uses three ingredients, oat water salt. They charge a premium for it, but it’s the best tasting one I’ve found.