When most fuels burn, water and carbon dioxide are the two main products. why can´t you say that water and carbon dioxide are products of all combustion reactions

Because some of the processes are also known as incomplete combustion.

Explanation:

Combustion reaction is defined as the chemical reaction in which a hydrocarbon reacts with oxygen gas to produce carbon dioxide gas and water molecule.  The chemical equation for this reaction follows:

If supply of oxygen gas is limited, it is known as incomplete combustion and carbon monoxide gas is also produced as a product. The chemical equation for this reaction follows:

Thus, we cannot say that all the combustion reactions produce carbon dioxide and water as products because it depends on the supply of oxygen gas.