Because it has nothing to do with Arabic. /p/ in Persian is aspirated, and in some words, like aspirated /p/ in some other languages (e.g. Greek), it has turned into /f/; Ever wondered why ph is pronounced /f/? In Persian this is called "softening" (Narm şodegi).

Not only just a superficial change, and if anyone calls it parsi in iran they just come across like a literary person, it’s that Classical persian literature as we know it is from after arab era until 1300s. Persian imported arabic words and used them widely in its literature. Maybe an analogy is english should be called Anglish.

The difference between Persian and Farsi matters in english world because it is political. In english, Farsi doesn’t carry prestige, same way iran doesn’t carry Persia. But they do for those in iran. I’m not familiar enough with afghanistan to know first hand, but perhaps same can be said for Dari. I might be wrong though on this last one.

Both are used in Iran. Though a common folk etymology, Parsi didn't change under Arabic influence. Words like abzar and afzar exist in similar variation, guwspand gufsand, ispand, isfand, Espahan, Esfahan. Even modern loans from Russian sometimes undergo this change like apelsin->aflesun.

There are some people suggesting that, however at a small scale and not taken that seriously by many. What difference does it make? What about all the other words that underwent the sound change? Also, some nuanced people can keep languages and politics separate. The sound shift isn't even entirely clear to be due to arabic influence, how come it turned into 'f' and not 'b' such as the arabic approximation? How come sounds like 'g' remained?

And in the end, in English it should be "Persian" and not "Farsi", that is where the actual move should be. How sad and historically wasteful if we started to do that to all languages, "deutsch", "zhongwen" or "elliniki" instead of German, Chinese and Greek

It used to be called Persian in English, the media changed it to Farsi to reduce it's "prestige". If you knew English and you are old enough you even remember the shift (1990s–2000s).

> the media changed it to Farsi to reduce it's "prestige"

This is not true.

It happened after the 1979 Iranian revolution, when Iranians abroad wanted to call it Farsi out of cultural pride, using the same word in their own language, rather than Persian which is the "foreign" word for it (from Greek/Latin). It was literally reclaiming the name. Then the media followed suit out of respect. It was cultural sensitivity.

Today some non-Iranians and therefore groups like the UN prefer "Persian" because variants are also spoken in Afghanistan and Tajikstan, and Farsi is a reference to the Fars province of Iran, so Persian can be seen as more neutral. But then again, not many people complain about "English" being associated with England and not being neutral enough to Americans or Indians. So it's definitely complicated. But it's also definitely not about trying to diminish anybody's "prestige".

I disagree (but also with the prestige theory) it's not really a thing in other countries with significant amounts of Persians such as Sweden or Germany, there the exonym is still more dominant and no-one tries to push "farsi" out of pride or anything.

I think it just was a random occurrence / perfect storm in the US where some Persian speakers moving there didn't know there was an English word for it, and where the local population were more used to hearing names of unknown languages, and it happened to stick around and start spreading. And _then_ it maybe became a cultural pride or whatever thing with media following suit like you said

I don't know how much you and I both care about this, and arranging the evidense is going to take some time. Specially since I don't know if I'm going to live in a warzone or not starting tomorrow. Sorry.

You might be interested in this https://en.wikipedia.org/wiki/Parsis