> I don't know why we would apply reductionism something as pleasurable and healthful as food.

I applaud you for admitting your ignorance. Let me give you some reasons why:

I know a guy who doesn't enjoy eating. For him it's always been a chore. He has a poor sense of smell, it's surely related. It works for him.

I drink soylent as an alternative to eating out on my own, which is an expensive and unhealthy habit I have. It does not replace 1) eating out with friends or 2) cooking at home.