Why are these resistors considered in parallel in the DC equivalent (Com Emitter Amp)
I think what you are missing is that they are looking for the Thevenin circuit from the base of the transistor to ground.  Redrawn, it looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The Thevenin equivalent for this circuit would look like this:







simulate this circuit

When calculating the value of Rth, you have two options:


Using the superposition theorem, treat the voltage source as a short circuit and calculate the equivalent resistance between the two points.  In this case, you have two parallel resistors.
Knowing the short circuit current (which is the current in the circuit through R1 if R2 was shorted), calculate what Rth needs to be to achieve the same current using Vth instead of Vcc.