Because the Chinese government (Mao) refused to acknowledge that it was happening. He surrounded himself with yes men so probably did not even know himself until it was too late.
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In addition to the historic reasons mentioned in the other comments, there’s places facing famine right now that aren’t getting much help and several more countries where we should be preparing for mobilization and aren’t.
Source for that image is the World Food Program’s early warning report: https://docs.wfp.org/api/documents/WFP-0000153539/download/
The PRC actually refused international aid. Mao was... not the sweetest fellow in the candy shop.
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