why phasors are treated as complex numbers?

The simple answer to this is that the rules for the addition and subtraction of phasors are the same as those for the addition and subtraction of complex numbers. Therefore the mathematics which have been developed for handling complex numbers can be applied directly to phasors.

Actually, I suspect that phasors were originally developed specifically so that complex arithmetic could be applied to ac circuits.

The underlying theory is this -

The basic elements from which ac circuits are formed are resistors, inductors and capacitors.

These have defining equations of these forms -

resistor: V = I*R

inductor: V = L dI/dt

capacitor: V = (1/C) integral I dt

If the current is sinusoidal in form, I = A sin(wt), then for a resistor, the voltage is also sinusoidal, but for an inductor, the voltage is a cos function since d(A sin(wt))/dt = Aw cos(wt), and in the case of a capacitor the voltage is a -cos function since the integral of sin is -cos.

But cos (a) = sin(90+a) (using degrees)

So the the voltage developed across an inductor with a sinusoidal current can be represented as a sinusoid phase-shifted by 90 deg. This rotation by 90 deg is exactly what happens to a vector when it is multiplied by the imaginary unit vector i (or j in electrical terminology). So phasors can be seen to be examples of vectors or other complex numbers plotted on an Argand diagram. This is of course extremely useful !