It's likely because other countries didn't know the extent of the famines.

Perhaps Mao Zedong himself was fooled by false, exaggerated reports. Perhaps he was desperate to hold onto his own powers against challenges like from more pragmatic officials like Peng Dehuai at Lushan. Perhaps Mao was trying to absolve any existing debt to the USSR asap due to the beginnings of the Sino-Soviet split.

In any case Mao insisted there was no problem until 1960 when Zhou Enlai confronted him (first crop failure was 1958), and even then would not comment about the extent of the problem (if he knew at all). And he insisted grain exports to continue and the repayments to the USSR -scheduled in return for previous economic and specialist help- to go on ahead. While many Chinese were starving to death.

The USSR did not know something was wrong until 1960. And even then they could not find out accurate information. The Chinese weren't telling, and any inspections they took they saw only Potemkin villages designed to "show" how well the Great Leap Forward was working.

Lorenz M. Luthi The Sino-Soviet Split