Why is all water available on earth not usable?

97.5% of the earth's water is salty, and it is very expensive and tricky to desalinate (get the salt out of) salt water in large amounts. 1.7% of all water is frozen in the earth's icecaps, so it can not be used either. 0.7% is ground water, which we can access to a certain extent, and only 0.02% is from lakes, rivers, dams, etc., which isn't much!