You are taking about what people eat and I am talking about what our body does with what we eat (what it is broken down into as it is digested). It is the only source of energy for our brain and red blood cells. It also provides energy to other cells too but is not the sole source. Of course our body needs other nutrients, but the brain is especially dependent upon it. Our digestive system breaks down foods into glucose (and other things). Your example of Keto and other diets talks about what is eaten, not what our body makes from what we eat. If you ate no sugar (and there is "natural" sugar in many foods such as fruits), because your body breaks down food into glucose, you will still have glucose in your system. Of course consuming too much sugar is problematic and the diets you talk about help people not do that, but our body also makes glucose from what we eat, including things that don't have sugar in them.
This article talks about some of this in layperson language (along with other common issues including consuming too much sugar)
