Why do we not include water in equilibrium expression calculations?
Usually when you have water in a reaction it is taken in excess, i.e in a much larger quantity compared to the other reactants. Ultimately the change in concentration of water is negligible. As an example you can consider the hydrolysis of ethyl acetate to ethanol and ethanoic acid. For better understanding consider the initial amount of ethyl acetate to be 0.01mol and that of water as 10 mol.
This however is valid only when water is taken in excess. If the amount of water and the other reactants is comparable, then water has to be included in the equilibrium expression.