Funny thing: in a semi-arid or arid climate humble mud bricks make a dandy construction material but where there are annual monsoons ... not so much. The subcontinent may have simply had a reason to more generally adopt stone construction earlier. Even when dealing with Egyptian civilization most of everything is still buried, so who knows what is under the Sahara?
The monsoon doesn’t impact the Indus valley all that much, particularly compared to some of the other regions of the Indian subcontinent.
Good point!