Here's why:

* Complete combustion means that the fuel reacts with oxygen to produce the maximum amount of heat energy possible.

* In this process, all the carbon atoms in the fuel are oxidized to form CO2, while all the hydrogen atoms are oxidized to form water (H2O).

* In addition to CO2 and H2O, complete combustion may also produce small amounts of other byproducts, depending on the fuel source, such as nitrogen oxides (NOx) and sulfur dioxide (SO2).

So, while water is a significant product of combustion, carbon dioxide is the primary and most abundant product of complete combustion.