It's because people are different, sometimes on a genetic level. What works well for one person does not work well at all for another, like the parent commenter, I do not work well without carbs. Keto puts me in a low-energy, semi-depressed state, while eating +80% of my calories as starch led me to lose about 15lbs of fat and now I'm the lowest body fat % I've ever been in my adult and teen life and my HOMA-IR went from 3 to 1.6!!!!
And my genetics also showed this to be the case too!
It got a bit complicated; it was a long research project, and it led to me starting my current company, Patchwork. Currently, it's at a few hundred. https://www.patchworkfood.com
This is crazy, I have the completely opposite effect. On keto + calorie restriction I feel much more alert and sharp.