> Which means that either there is no such an internal pressure or the regimes are extraordinarily good at selecting and incentivizing people to maintain the status quo.
It depends a lot on how much power the people have. The more advanced and diverse an economy and the more qualified and educated the population are the more power they have. On one extreme you have countries like Angola with an economy consisting virtually only out of exporting oil. These countries only need a few qualified engineers for their resource extraction which they pay well and everyone else is entirely replaceable. That leads to extreme inequality between the leading political class that absorbs all the money and pays the military with it. As long as they pay and treat the military well enough they can just suppress the rest of the country. If people act up they can literally just kill everyone part of the rebellion. The political class, the military and the rest are just entirely disjunct classes of people with different incentives. The family of the militaries profit enough from the system to not excert pressure on their family member working for the military. It's the hand that feeds them.
On the other end you have countries with highly developed, specialized economies and a population that is educated enough to understand at least a few things about politics. There ordinary people have extensive training and work experience. You cannot just replace them. They can protest and go on strike and if you start killing everyone the economy will quickly start crashing down. Just pulling a few cogs out of the massively complex machine will stop it from working. And at that point it's not just a problem for the working population but also for the owning class and the pressure will propagate all the way up through the hierarchy. Also people can just leave. They have the economic means to and their qualifications mean that other countries have an interest in attracting already qualified people without having to pay for their education and traning first. That's what happened to east Germany and why they built the wall.
There are some methods of social control that can help to control a population beyond that. The key ingredient is surveillance, mutual control and seeding distrust. One person alone can never challenge the system. People need to organize. You can try to find the organizers via surveillance quick enough and get rid of them before they get dangerous. Also if a significant portion of the population is secretly informing the government people might be to afraid to organize as they distrust each other. That's how the Stasi worked in East Germany. For an extreme case of that see the Inminban[1] system in North Korea where people are bundled into groups where all surveil each other and report any dissident behavior. Failing to do so will lead to collective punishment for the whole group. It's a really perverse system that plays people against each other and their own interest aligning the incentives for the individual with the government rather than their class.
[1] https://en.wikipedia.org/wiki/Inminban