Why is load resistance neglected for an ideal op-amp?
In the image below, for ideal op-amp, output resistance is zero.

You've said it in your opening statement. Due to the feedback the op-amp will adjust the output to maintain the desired output voltage.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. The equivalent circuit.
The output impedance and the load form a voltage divider and we can apply the formula
$$ V_{OUT} = V_1 \frac {R_2}{R_1 + R_2} $$
Since the output impedance is zero this becomes
$$ V_{OUT} = V_1 \frac {R_2}{0 + R_2} = V_1 $$
so
$$ V_{OUT} = V_1 $$
Since R2 doesn't appear in the equation it has no effect on the output.