I never said its a religious term. Plus why is it so hard for Persians to pronounce Sindhu as indu or whatever.
Accent.
The Indo-Iranians, when they split some 4,000 years or so ago, split down the middle with regard to accent as well. On the one side, they had difficulties with the sibilant, 's', and tended to make Hindu out of Sindhu, Haraovati out of Sarasvati.
They also had theological differences, and thought of Daiva as malignant, Ahura as beneficial. Those who split from them and took an easterly path, traversed the grim mountains and debouched into the plains beyond thought the opposite; so in the Rg Veda, Deva is godly, Asura (replacing the Iranian aspirate with a sibilant) was a demi-god who was on the other side of the Devas=Suras. This concept of two or more sets of demi-gods is not unique to Indians and Iranians; the Greeks had them, the Gods and the Titans, and so did the Scandinavian/German tribes; their leader in battle, the God Odin, was Aesir (Asura) and fought the Vanir (not clearly identified in Indian myth, but I have my views).
It is not hard to pronounce; it is due to habit and custom.
sir, I'm talking facts which you fail to realize or your just trolling.
Why is it so hard for Persians to pronounce Sindhu as sindhu when they everyday pronounce words like Shia Sunni etc.
You are not talking of modern Persians, but their very remote ancestors; the usage of those ancestors came down unquestioned. If you have learnt the term Arab, and somebody comes to you and says that the name should correctly be H'arab, would you change?
Pakistanis also call India, "Rapistan" "Tioletlessistan".
We know what to think of such creatures, whatever their nationality.