It’s Because Many of the Pioneers and craftsmen in Bollywood came from the Urdu medium.
It is important for modern day Indians and Pakistanis to understand that
the Hindustani language spoken in olden Bollywood was the language of formal education in most parts of India the Nastaliq Script used to write Urdu was popular among major parts of North and West Undivided India because of Mughal reign heavily emphasized on use of this language as it was the language of people as compared to Upper-class Indians who spoke Farsi in place of Urdu.
Interestingly up until sometime in history until last 200 years Urdu with the nastaliq script was regarded as the language of Hindus and labelled as “ language of Hindus”.
until the British came and created a santadardized version of both the languages and divided them into Hindi and Urdu respectively and post partition both the countries started using them creating the language divide further, the popular saying
Almost most of the yestyear stars like Balraj Sahni, Raj Kapoor, Sunil Dutt , Dev Anand , Dilip Kumar Dharmendra and Manoj Kumar used to read and write there dialogues in which now is regarded as Urdu.
Hence these two languages have there separate exisistence as of now but deep inside they are one and cannot withstand without eachother’s coexisistence just like our Nation
Namaste