Why don't we consider the voltage drop due to inductance of conductors in dc motors?
We do. The mathematical model of a DC motor includes both armature resistance and armature inductance. These two terms give rise to an 'electrical time constant', which is small compared to the 'mechanical time constant', so the electrical one is often ignored. Electrical time constant is \$\tau_e = L_a/R_a\$, and the Mechanical time constant is \$\tau_m = J/b\$ where J is the motor inertia and b is the viscous damping constant.

The reason that armature resistance, alone, is not ignored is that it has a significant effect on the steady state speed. Armature inductance only affects the transient response, and that only slightly, as the mechanical components are dominant.