Because if you spend enough time on the internet, especially the Western side, you start to hear voices of people who oppose their governments and not enough from those who support it.
Even many who are supportive tend to say that their government is authoritarian.
China: very very high, maybe 90%, their material conditions have progressed in leaps and bounds. Most people want a better life for their kids: the government is delivering that = most people are satisfied with the government’s performance
Cuba: much more mixed, people are suffering economically. Says you, “Oh but that’s the fault of outside influences”… well your particular explanation of the causes will chime with a certain percentage of the population only, there’s lots of angry anti-government people in Cuba, but they may not be a majority, hard to put a number on it
Vietnam: I have no info on this