Porridge is different. It’s been cooked which crosslinks the starch molecules. Oat milk isn’t porridge water… it’s different down at the molecular level. Believe me, I made this mistake when ‘cooking’ up a batch, sorry, that should have been ‘cocking’
I looked into the high price of plant milks. It’s essentially because the industry is new and still investing in R&D and new factories. The dairy industry has very little innovation now, just court cases.