It's cliche for a reason: they want to make movies about how people survive in a lawless, violent and chaotic setting. Therefore the aesthetic has to represent that: marginal, vaguely threatening people wandering around a ruined landscape, with the clothing and manners to match. Basically, after civilization falls, the only people who will be left are those who rejected society in the first place. Plus, if all the extras look threatening, it's a cheap and easy way to create drama.
For what it's worth, Thunderdome did indeed present some "ordinary people" who established a colony at the end of the movie, if I recall correctly.
Otherwise, I do agree that it's very plausible that people would be, overall, rather less lawless if civilisation fell.
Conversely, I think that re-establishing the modern day would be more difficult than one might think: if I'm not much mistaken, a fair bit of our modern world relies on large, complex chains of supply and expertise that might be very difficult to rebuild in such a scenario.
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I think it would resemble the American West. Low tech, low density, but connected by commerce and family.
I suspect that a post-apocalyptic world would end up becoming a more agrarian society.
Without the industrial base and complex international supply chains, agricultural production per person would drop, resulting in a much higher percentage of people needing to be directly involved in food production.
It probably wouldn't reach the 9:1 ratio of farm laborers to everyone else we saw in many pre-industrial societies, but it would be considerably higher than it is today.
It always annoyed me that with the Mad Max franchise and its many clones, rip-offs, etc, that it's always bikers, mutants, bandit kingdoms and so on that exist in these settings. Where are all the normal people? The average person has some level of intelligence and survival capability.
And what is up with society regressing long-term or even permanently in the event of the world ending? It's almost a cliche that humanity collapses into an agrarian society (like in Threads), Medieval Stasis (like Shannara) or even a primitive Stone Age existence! If the world came to an end in 2008 (nuclear war, World War 3, zombie apocalypse, the Plague, etc), the survivors would try to rebuild and recreate 2008 as closely as possible. They wouldn't start from the Stone Age and work their way through Antiquity, the Middle Ages, the Renassiance and all that. They'd look for any media with such information like books, audio, video, experts and so on. If they didn't succeed, they'd leave that to their progeny to rebuild the world.
It is hard to enjoy post-apocalyptic media with those cliches.