Why does not all the current flows through Zener diode?
No, you cannot replace a zener diode with a resistor. You can replace the diode with a voltage source, equal to the zener voltage, in series with a small resistor. But this model only applies if the zener is reverse biased, meaning that current flows into the positive terminal of the voltage source.

In your second paragraph you say "the current will flow through it [the zener]" but it is more accurate to say "some current will flow". Some current also flows through R16, because it has a voltage across it (the zener voltage).