Why is PTSD considered Anxiety?

PTSD is a subset of symptoms that persist following a traumatic event.  It is characterized by three main components:

1. Re-experiencing the trauma through intrusive distressing recollections.  I.e., constant thinking of it, nightmares, flashbacks.
2. Avoidance of reminders...people, places, activities.
3. Increased arousal such as inability to sleep, difficulty concentrating, feeling on edge or jumpy, irritable, angered, etc.

Anxiety has very similar symptoms, although there does not have to be a traumatic event to experience anxiety.   Anxiety can be characterized as follows:

1. Rumination or constantly being stuck in your head, thinking of a situation over and again attempting to figure out how to have a different outcome.
2. Avoidance of people, places or activities that elevates anxiety.
3. Poor sleep patterns, concentration, and irritability or edginess accompany.

And of course PTSD not only comes with baggage of anxiety, but also depression!  There's that vicious cycle again.