Most markets, in terms of their daily volume, are open at night, but very thinly traded until EU hours, but some do see action in Asia hours. It’s just about liquidity.
Maybe you’re thinking of single name equity markets, which are a fraction of daily trading.
I think they’re probably asking about US markets. Afaik those do close at business hours and I’m not sure how after hours trading happens but it might not be available to most people.
I think the one of the main reasons is government concerns about shenanigans happen overnight without oversight / flash crash. That being said I presume all the breakers that would halt trading activity are probably automatic but potentially not all of them (I think the US did things like that during the housing collapse).
