Because they were clever. It reminds me of what I was taught in primary school physics class. Matter cannot be made neither can it be destroyed.
The reality was the seeds of Pakistan or Bangladesh existed. That seed was demographics. When you have over 60% demographic majority in such large populations over time nothing can prevent them from settling to their centre of gravity. Balochistan, K-Pk, Western Punjab, Sindh had such massive Muslim majority that they would have gone their way eventually. So would Bangla. Nehru knew that it was better to carve it then under Mountbatten's apologetic purview then accept Cabinet Mission Plan and lose all of Punjab as would have happened. I ony wish Nehru had agreed with Cabinet Mission Plan. Today we would have had a better Pakistan.
And tell me when GM Syed pased the resolution for Pakistan making Sind the first brick in that project where did it have a caveat that they were going to accept 10s of millions of migrants from Bihar, Utter Pradesh etc?