There's one thing that always baffles me about this kind of market work. Let's for the sake of argument assume that HFT and other sophisticated market making activities are crucial for price discovery and other great social benefits. Then why does this amazingly important social good get mostly turned off over 80% of the time[1]? Even as a retail buy-and-hold investor in boring ETFs not being able to trade outside normal office hours is an inconvenience. Surely the world economy has even more uses for trading at all hours than me?
Without arguing in favor or against the usefulness of HFT, the reason trading hours are limited is to increase liquidity during the specific hours that trading takes place. Liquidity is important for various reasons, in particular it helps reduce spread and thus there are better prices. Trading during other hours is possible, either during pre-market or after-hours, and there are even exchanges that enable trading on weekends, for example https://www.ls-tc.de/de/faq . During weekend trading, the spread is significantly higher.
I will say up front that I don't think the social good is worth what we are collectively paying for it, but I do think the market hours are a reasonable device. This is basically because there are humans involved and they need to sleep (Matt Levine has written about this).
If you want the best price, you need to have all of the market participants bidding together. Market hours serve as a coordinated period in which ~all market participants agree to be online and bidding.
Prices, thus, get stale overnight. But we assume that that is mostly okay, as business is normally conducted during business hours, and we assume that transactions can wait until the next day. ACH transfers take multiple days! (technically so do stocks, but that's mostly invisible to retail traders).
If you're a retail trader, I would caution you somewhat against trading after-hours; there is very little liquidity and it could cost you 100s of bps more.
You can trade outside of market hours just fine and many products (e.g. E-mini S&P 500) trade all night. It's just that a lot of companies publish news right after the market closes so prices are more volatile. And people sleep or play videogames at night so there's less liquidity.
[1] https://www.nyse.com/markets/hours-calendars