Why do we often write the water product as a gas in complete combustion reactions?
Well, let's put it this way.
Combustion reactions are generally quite exothermic. That means it releases heat. We can represent that like this:
#"Hydrocarbon"(l"/"g) + "O"_2(g) -> "CO"_2(g) + "H"_2"O"(g) + Delta#
where the hydrocarbon is generally a liquid if it is massive (larger than butane, usually), and is generally a gas if it is small.
#Delta#on the products side would represent the heat released due to the reaction itself.
Since water is produced, the water can immediately absorb that heat.
The heat has a greater effect on the water, since the liquid water molecules had less energy to begin with than the gaseous
It is true that in a closed system, water vapor could condense onto the insides of a bomb calorimeter, but we usually just talk about the moments immediately after the reaction occurs.