Why Iranians get angry when I call them Arabs ?

Simply because they are not Arab. They are Persians. 

I knew this one gal at college who never wanted to be referred to as "Iranian" but wanted the title of "Persian". I understood later why. 

Iranians are Persians.   It is like calling someone from Japan a Chinese.  

 Umm 🤔, probably because they’re not?! They are referred to as Persians.

Why they get angry, I don't know.

Iranians, ethnically, are Persians, not Arabs. Furthermore, although the language of the Iranians (Farsi) uses the same script as Arabic, it is different linguistically.

Not too bright, are ya?

Iranians speak Farsi, not Arabic.  Iranians are Persian, not Arab.

Because they aren't Arabs.  And what they are talking is Farsi; it is not Arabic.

Possibly because they have been at war with the Arabs for a couple of millennia.

Your post has no relevance to SF or travel.

Iran is the current name of the ancient country of Persia. Most Iranians call themselves Persian.

Iranians are not Arabs.  Arabs come from Saudi Arabia.   

There is a longstanding history of conflict between the two countries dating back to 637AD.

The conflict today is primarily economic and religious as the two countries practice different forms of Islam.

Given the tone on your post I would imagine that most people who encounter you could be easily angered by your ignorance and arrogance.

Because they are Persians and they speak Farsi.

It's like calling a Chinese person a Japanese.