$$ecause$$ We know that reactance due to inductance $$X_{L} = 2pi vL$$. As frequency is high so $$X_{L}$$ will be high or $$L$$ can be considered as open circuit for high frequency of $$ac$$ circuit. In similar way, $$X_{C} = dfrac {1}{2pi vC}$$ At high frequency, $$X_{C}$$ will be low. So reactance of capacitance can be considered negligible and capacitor can be considered as short circuited. Here the above figure is equivalent circuit to given circuit. Total impedance, $$Z = R_{1} + R_{3}$$.