PPP is inherently tied to the basket of goods, but the baskets aren't standard. Even if they had the same products in it (they don't), products themselves can vary a lot. Some yogurt can be made from real milk made in a clean factory or can be a fake based on palm oil mixed in a dirty garage with rats and no health controls.

It also includes the cost of housing, so you end up comparing building-controlled and highly regulated areas like Bay Area with who knows what in India.