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.
Dirty Westerner, but from polls and outside forms of public sentiment I have seen, public support of the government in China is extremely high, due to the large, until possibly recently, economic growth and decades of constant improvement in people’s material conditions.
Cuba it is not as strong, as the US blockade has prevented the strong growth China has experienced. But there is still majority support due to the Batista regime still being in living memory and the various improvements in literacy and education, medicine/life expectancy, and various forms of local democracy. There are organic and legitimate critiques, but anti-government protests are usually dwarfed by pro-government/M-26-7 protests. And usually, though this is often misrepresented by pro-western sources, the dissatisfaction with the current government is more pro-reform rather than in favor of reversion back to capitalism. Though this is one of those things that polling or any way to quantify public sentiment can very easily be twisted with framing.
Vietnam, I have less knowledge of and cannot give a summary off the top of my head with any trust in my own knowledge.