The 23/7 is not so much for maintenance as to have a defined window for changes to the market to happen.

Every so often a new stock is listed or a stock ticker is changed or a stock is split, etc. There are smaller changes every single day, like to the settlement date of your trade.

It's very convenient to be able to restart all your systems at 5pm, have them all load the updated reference data, and start them again in time for 6pm (or 7pm, or 4am tomorrow...). Even if you trade stocks and options and currencies and futures all over the world, a quirk of the calendar means they're basically all closed between 4 and 5pm Chicago time.

Of course it's possible in principle to build systems where all this is dynamic and you can seamlessly trade with the old configuration at 4:59:59.999 and start trading the new one a millisecond later. But literally everyone has built systems that don't work on this, that rely on being able to chunk the continuous passage of time into discrete days. It would be painful to rearchitect them all now.