Acommon health myth is that people should drink at least eight glasses of water per day while staying hydrated is important why could this advice be inaccurate for different people

This advice could be inaccurate because each individual is different. We belong to the same specie, we have similar but different necessities. All humankind have the need of eating food and drinking water, but the amount of each one variates one person to another, and there is the difference.

The amount of water a person must drink depends on health, size, amount of fat, if it's a sedentary or an active person, all these and more variables determined the healthy habits a person should take. Just taking a specific diet from one person and apply it to another, it will lead to disaster.