why is parasitic inductance of secondary coil is ignored
The parasitic inductance of the primary coil can be said to be formed by those turns that don't effectively couple their magnetic field to the secondary therefore those few turns form what is called a leakage inductance and is regarded as equivalent to an external inductor in series with the "ideal" primary.

However, that leakage inductance (as a single component) represents both the  primary turns and the secondary turns that don't couple magnetically. In other words, the primary parasitic inductance value that you might read in a data sheet represents (in one component) the leakages of both primary and secondary.

Some data sheets will specify both leakages but they can be grouped mathematically (and quite easily) into one component at the primary.