It seemed to me that about 70% is generally the average figure for the water content in the body as a whole. It is not entirely clear what the author of the question means: does he understand that the percentage of water content in most other organs is close to these figures? If we turn to the question of the origin of biological systems and apply elementary logic, the question easily ceases to be a question: molecules that have become components of living systems as a result of chemical evolution appeared in a liquid medium, some lipid molecules, amphiphilic in nature and capable of existing in water only in the form of micelles, they formed something like a membrane for protocells, and the "primary broth" contained in them, the main component of which, as far as I understand, was water, formed something like a precursor of the cytoplasm. Further, in the course of chemical evolution, the composition of the contents of these micelles gradually became more complex, but water remained the main component. Billions of years later, when the first cellular organisms appeared - and they appeared from colonial unicellular organisms, the water that washed these colonial progenitor organisms, when they (colonial) "closed" into a multicellular organism, became the basis for the internal environment of organisms - that which during evolution became blood, lymph, cerebrospinal fluid, tissue fluid.
In general, there is a lot of water in the brain (and most other organs), because it forms the basis of both the cytoplasm of cells and the internal environment that bathes the cells, and initially there is so much of it in the cells and the internal environment of the body. because see above.
In order to understand why there is so much water in the brain, in principle, it is enough to drive into google "the role of water in a living organism". Supporting the previous speaker, I will say that the brain really contains a lot of lipid structures. But for the proper functioning of these hydrophobic structures, an aqueous environment is required. Also, most synapses are chemical or mixed, and water is necessary for the transport of neurotransmitters.
In the brain, and indeed in the central nervous system, there is a large content of water in the tissues. I don't know how much%, but more than in other fabrics. Proof - colliquation necrosis occurs in the central nervous system. Why is there a lot of water? The brain washes the cerebrospinal fluid, it is needed because some vessels cannot cope with the energy and plastic needs of all neurons.
I disagree with the idea that the brain is made of water, because most of it is made of fat - these are fat cells. The brain just does not need so much water and, even more, if too much water accumulates in it, then a serious condition occurs, which is called cerebral edema. The brain does not need water in the amount you are talking about.
Let's look at the simplest example. For example, with alcoholic intoxication, cerebral edema occurs, because being saturated with alcohol molecules, the brain pulls water there - each alcohol molecule chemically attracts five water molecules, due to which edema develops, forming the notorious state of intoxication.
The second thing that often happens after drinking alcohol is mechanical head injuries, which also lead to cerebral edema, no matter what mechanism this injury is.
The third situation in which the amount of fluid in the brain will increase is an infection. Such as, for example, the flu. Viral infections can lead to impaired blood flow and cerebral edema.
Edema can be fatal, but for this the edema must be quite intense. And not only in terms of volume, but also in terms of the rate of growth, because the body is a self-regulating biological system that itself copes with various problems. But if everything happens quickly, then it can lead to death - neuroinfection, impaired blood flow, rupture of blood vessels, in which blood is filled. Blood, on the other hand, is a liquid, in which almost 80% of water will be. And these are already fatal consequences for the brain.
In fact, the percentage of water in nerve cells is not necessarily 78, usually indicate something in the range of 70-80%. Water is part of the intracellular fluid, and not only in the brain, but throughout the body. The inner parts of the cell (organelles) seem to float in an aqueous solution. Neurons also need water for the production of hormones and neurotransmitters - special substances necessary for the transmission of nerve impulses from one brain cell to another.
In addition to the water contained in cells, inside the brain there are real lakes of cerebrospinal fluid - cerebrospinal fluid, also being an aqueous solution. This fluid protects the brain from mechanical stress, being a kind of shock absorber, and supports metabolic processes between the blood and the brain.