Just because a market is open does not mean you have to trade. It could be good for the economy. Have a nightshift Goldman Sachs for example. All those laid off wallstreet folks could work the night shift and others do weekend rotations.
There is a lot of legacy stuff from the pre-computer era, not just market hours. For example settlement could nowadays easily be instant instead of tom/next, or interest payments could be realtime instead of o/n. It would even make sense. But I guess its nigh impossible to get everyone to agree to new conventions (not to mention expensive IT upgrades and so on).
You have 4 hours of pre-market and 4 hours of post-market trading, you can buy and sell stocks and ETFs 14.5 hours a day, that is enough for most people. On the CME/Globex you can trade futures 23 hours a day.
It is almost 24/5.. however it is still not 'all computerized', you still need tech support people to troubleshoot the computers as software/hardware/networks are not 100% reliable. At the weekend, lets say if some major black swan news event comes out.. the markets go crazy and you only have token crew at the exchanges and at your brokers.. they wont be able to handle any problems. Also the market makers probably arent interested in making markets at the weekends for similar reasons.