Because our national narrative is based on being "Citadel of faith", and not Citadel of knowledge.
When I can be content and have no identity crises as a Muslim while accepting my Hindu/Buddhist ancestors, why can't other Pakistanis who still cling on to myths of being Abbasi, Umayyads etc do the same?
Arabs still remember their pre-islamic prominent personalities and celebrate them, while we are taught by our religious clergy that being a Hindu/non muslim is unclean etc and how brave were the Central Asian and Persian looters who destroyed and butchered our ancestors and desecrated religious places of worship, no wonder why we see so many Pakistanis here and other places making fun of Hindus whenever India is mentioned, forgetting not only that there are millions of patriotic Pakistani Hindus, but also, more than likely, their ancestors themselves were Hindus*
Seriously confused people, trying to adapt a foreign narrative. Just be content with what you are today and teach your true history instead of inventing fake family trees and origin myths.
* I know Hindusim is a syncretism of different faiths, philosophies etc.