I can tell you why, though I am very much against Soylent: convenience. My day is limited, such that I spend a significant portion of my free time cooking. If I had those 2 extra hours out of the 24 back, I feel I would be happeir and more productive. This is the same reason I wish I could get away with only sleeping 2-4 hours a day (I tried at one point to mess with sleep cycles; it was not fun).

Why not Soylent? Because it will kill you. It's pretty simple, I think. I am a big fan of Michael Pollan, and his main argument is that even nutritionists and doctors, people who dedicate thier lives to studying nutrition, can't answer lots of the more complex questions: "Eat carbs! No, wait, eat protein! No, eat whatever you want, just get enough vitamins! Fast! Eat constantly!" What are the chances that a single non-nutritionist got it right? I think exactly 0, and I'm not willing to prove that theory wrong using my own body.

Edit: ideal situation is where the human body naturally requires only 1-5 meals per week. Then I could easily see myself cooking a few gourmet meals. Alternative: be so filthy rich that you are either less busy or have a chef ready to cook for you any time of day.