More APIs than not on this planet are considered business critical and doesn't just close. Stock markets are strange as they must be highly available for a few hours and then can be shut down for the day. Imagine if AWS closed every night.
A lot of Amazon retail does close every night. You can put in a request but it won't be fulfilled and shipped until morning. Getting rid of that is a nice idea, but also "instant fulfillment" is not great either without "instant returns." The idea of "instant trading" seems like it can instantly put the system into a bad state with no recourse to fix it.
The real reason stock markets close is because financial institutions are very interested in keeping it that way. It is lucrative to play pretend stock markets after hours.
(This is a very real question all over Europe right now, and it is interesting to see which institutions talk the loudest about how beneficial a shortening of opening hours would be to just about everyone.)
Shouldn't the question be "why not"?
More APIs than not on this planet are considered business critical and doesn't just close. Stock markets are strange as they must be highly available for a few hours and then can be shut down for the day. Imagine if AWS closed every night.