Harrapan civilisation is NOT Indian. It may be described as "subcontinental", "belonging to the Indian subcontinent", or "south Asian", or "south Asiatic".
Calling it "Indian" confusingly associates it with the most brilliant most secular bestest republic of India, which is an erroneous association. When this particular very very secular republic was born in 1947, they relinquished any putative claim that they may or may not have held over the land of the IVC.
The Harappan civilisation is an ancient Pakistani civilisation, or plausibly, a "native Pakistani" tribe, though that may be debatable.
At no point in recorded history did Pakistan or any Pakistani government relinquish its ownership of the land and legacy of the IVC. Why should we allow the secular republic to adopt a legacy that was never sitting in some orphanage asking to be adopted?