Cryptocurrency markets are open 24/7 btw.

Everyone involved in a stock market agrees it's quite useful to have a separation between days, although you could have a 23.5/7 stock market with only half hour day-ends. That's when things like index rebalancing, addition, removal, stock splits occur. You say whoever holds stock X at end of a certain day holds stock Y+Z at the start of the next day (due to a company splitting up) and there's no need to argue about which millisecond a trade occurred. (It's your own damn fault if you were trying to buy the stock in the milliseconds before closing, but some exchanges run a special closing auction with a different algorithm to prevent races). It's also the time you can deploy software, which you can't do during the day. Maybe half an hour is too short for all this stuff. Or maybe it should be a 12 hour window once a week instead.

I think you can have a separation between days while still allowing 24/7 trading. Just schedule automatic operations like stock splits at midnight, and with a proper staging environment you can deploy software in the middle of the day, too (just give a heads-up first).

And of course, overnight trading is available on some stock exchanges now. (I’m not sure whether this is a new thing or has existed for a while, but I’ve only noticed it on TradingView for some papers about a year ago I think.)

And isn’t forex 24/5?