In case of baby milk powder the alternatives exist on the Chinese market. Even german brands are present in Chinese supermarkets. But after some high profile cases regarding low quality products[0], some people are willing to pay a decent premium for a product sourced from a European supermarket. On the height of this crisis I have actually seen signs "max 2 packets per person" for baby milk in german supermarkets close to train stations / airports. I actually met someone who financed a trip to China by maxing out his baggage allowance with baby milk.

[0] - https://en.wikipedia.org/wiki/2008_Chinese_milk_scandal