Why isn't auto ionisation of water considered when we find the pH during salt hydrolysis?
What happens in salt hydrolysis is enhancement of water ionization with salts. Try asking yourself where do $\ce{H+}$ and $\ce{OH-}$ come from in the solution of $\ce{AB}$, and you'll find that they come from $\ce{H2O}$. Or equivalently, hydrolysis of $\ce{A-}$ can be written as
$$
\ce{A- + H+ &lt;=&gt; HA,H2O &lt;=&gt; H+ + OH-}\stackrel{\text{add}}{\Longrightarrow}\ce{A- + H2O &lt;=&gt; HA + OH-}.
$$
In a word, ionization has been taken into consideration in your calculation. There are multiple equilibria in the system, so you cannot simply add $10^{-7}$.