Why don't we produce water from combustion?
The answer is in the formula.  The 'other substances', which are either volatile matter or ash byproducts, are formed along with the water.  The water is not pure because some of volatile substances can condense with the water which will require further treatment for removal.  Removal of the volatile matter and the disposal of the rest of the 'other substances' will cost too much money for this solution to be economically feasible.