From without it's also difficult to imagine toppling the North Korean regime. To do so would almost inevitably result in quite literally hundreds of thousands of dead South Korean civilians in a matter of hours (through artillery). And perhaps just as many dead South Korean, Japanese, Taiwanese, or American civilians dead through a nuclear attack. That sort of cold calculus makes it very hard to make the decision to end the DPRK regime.
North Korea's military could easily be beaten -- if you don't mind possibly hundreds of thousands of South Korean and Japanese killed by their missiles. You'd win the war but lose a battle no one would consider worth losing.
You'd also have to negotiate with China first, so they know you'll stop invading well before their border. Last time the U.S. incurred too far into North Korea contributed to China entering the Korean War.
North Korea effectively holds South Korea and Japan hostage with its missiles. It acts belligerently to make credible its threats to use them.
Meanwhile, the people don't know things could be better. They aren't stupid or brainwashed. The state simply controls nearly all information entering the country. They believe they have nothing to envy from the rest of the world and that the U.S. is starving them. From their perspective they have no reason to revolt.
They have no internet or knowledge of revolutions elsewhere in the world.