Sweet tea includes sugars which are bad for your body, water is totally pure and good for you.
Some water includes additives like fluoride that healthy for your teeth.
Water is usually cheaper then sweet tea.
Water is usable in a much wider range of recipes. Adding sweet tea to many things will ruin it.
Water will not go bad nearly as fast as sweet tea, tastes good hot, cold, lukewarm, can be boiled and still be tasty afterwards, can have many other flavors added to it at will and is a major ingredient in making sweet tea.
In effect water can be made into sweet tea if you desire it, but the reverse is not nearly as easy to accomplish. Therefore, since water can have all the positive effects of sweet tea (by adding things to it) and none of the negative effects, along with a huge variety of other possible uses, water is clearly superior.
Not everyone likes sweet tea, but everyone likes some form of drink that includes water.
Water is also generally better to drink (often heated and steaming) when you are sick and/or congested. Sweet tea would probably not be optimal in such situations and could even make the patient feel worse. Diabetics would need to carefully monitor blood sugar levels if drinking sweet tea but only gain positive effects from drinking water.
The list goes on and on.