Because it is nothing but bullshit propaganda.
I have written about this before on PDF but in short "India" never existed and the whole idea is a lie.
Pakistan is the offspring of one of the five great neolithic civilizations. It was and has never been a part of "India" (a name from my readings first introduced by Europeans after the river "Indus" which isn't even located in "India"). Hinduism never existed in Pakistan and the religion itself does not exist (it's a collection of folklore religions similar to what you wound have found in China grouped under a name derived from the word "Hind" which is Persian in origin). Furthermore, the Rig Veda is not a "Hindu" text nor does it have anything to do with India it was created by Pakistan's ancestry (not only does it espouse the idea of monotheism but the parts of it that are polytheist in nature are very similar to the religious views of the ancient Middle East and Greece not the Brahmanical religious views of India). Pakistan's ancestry has always rejected the caste system instead employing a class system, it ate meat and buried it's dead (confirming the belief in an afterlife) which is the total opposite of "India's" Brahmanical beliefs. This is further evidenced by the fact that not a single "Hindu" artifact was ever uncovered amongst Pakistan's ancient ancestral sites. Pakistan's ancestry accepted Islam not only because of our long standing relations with the ancient Middle East (particularly Ancient Egypt and Mesopotamia now Iraq) but because the religious views were already close to what our ancestry practiced.
There is absolutely nothing that ties our people together other than the Mughal Empire which was later replaced by the British Raj which India allowed into it's territory and sided with against Pakistani peoples (unlike us who fought against the Greeks whose empire collapsed within 30 years of venturing into Pakistani territory). Indians are the byproduct of two neolithic settlements one emanating from the Belan Valley (that later expanded into the Gangetic valley and around the Ganges) the other being the earliest settlement in India straight from Africa into Tamil Nadu. Pakistan's ancestry stems from the Mehrgarh that expanded into all of modern day Pakistan (including Kashmir) which held totally different views from your peoples in "India".
I can keep going on relating to this topic but want to stop short of turning a response into an essay.
India is a nation that was forced together. Hyderabad never wanted to be a part of India, you have a continuing movement for an independence in Khalistan, the Red Corridor are a collection of states that want independence, occupied Kashmir wants to be with Pakistan or become an independent state, etc...
This idea that Pakistan was artificially created is nothing but nonsense. It was destined to happen and no man could have stopped it.