It takes a certain condition for the body to make water out of fat. Generally, insulin levels have to be low. High insulin levels inhibit fat burning - and it’s the burning of fat that allows the creation of metabolic water.
Too many people don’t fast. They always eat or drink something throughout the day, so that the conditions that would allow for metabolic water to be made are not created.
For a while, I think Cole (snake diet guy) only ate bacon and cucumbers, and wouldn’t drink any water outside of that. I think he did it for a month.
The rate at which you produce water is proportional to your fat metabolism. The more fat adapted you are, the faster you’d synthesize water. Dehydration might simply be a case of the demand for water exceeding the supply, fat storages notwithstanding.
Slow fat metabolism might be why humans can’t exclusively survive on metabolic water like some desert animals can. Or maybe humans can and our metabolism just isn’t trained well enough (except me, I’m amazing).