It is because water is in excess.
At pH = 7 which is assumed to be normal pH of water, though it is generally more acidic due to auto absorption of carbon dioxide gas making acidic HCO3-, we can write find the concentration of water molecules expected to auto-ionize.
Water reacting with itself: H2O + H2O ---> H3O+ + HO-, remember that pH = -log[H3O+], so then:
7 = -log[H3O+]
10^(-7) M or moles / liter = [H3O+]
which is actually the same about of HO- that has formed from water reacting with itself at pH 7, so appreciably not much H2O has auto ionized.
Density of water at room temperature is about 1.000 grams / mL
Molecular weight of water is about 18 grams / mole.
Use that to convert to moles / liter by cancellation of units.
(1 grams / mL) * (1000 mL / 1 L) * (1 mole / 18 grams) = 55.56 M H2O
Which H2O is much in excess of the concentration of concentration of hydroxide and hydronium (across the whole pH scale like pH 1 to pH 14), so when you have a reagent in unreacted excess, you ignore it in your equilibrium calculations with respect to chemical kinetics.