They probably got covid.

The inability to recognise how an intelligence trait that correlates with pattern recognition might have an effect on being able to predict the long-term outcome of one's actions and provides the ability to work well with others in a very large group, which a civilization is, and instead defaulting to an ad absurdum argument shows where you might fall on the IQ spectrum.

I think I read that most ancient civilizations collapsed because of environmental factors. The environment changed, and they couldn't cope with it for some reason (lack of technology was probably a big part).

As for China, I don't think that one ever "collapsed". It withdrew, but I don't think you can call it a "collapse" the way many other ancient civilizations did. Even Rome is a bit hard to call a full collapse: it degraded over the course of centuries, and eventually Europe turned to feudalism. The Roman Empire itself split apart, with the eastern part (in Constantinople) continuing for another 1000 years after the western side fell apart.

I think civilizations like the Assyrians and Hittites are better examples of civilizations that truly "collapsed": there's basically nothing left of them today, except some ruins.