Why aren't after-hour and pre-market trading just part of the trading day?

I understand that once, trades had to be done through phone calls with brokers who had 9-5 jobs and such, but, in the modern world, most sources seem to say that anywhere from 60-80% of trades now happen online. So why are after-hour and pre-market trading still "separate" now; why haven't the markets made the trading day longer, say, 8am-8pm, or even just 24 hours? Similarly, why do they need to close for holidays and weekends and such, when most of these trades don't require anyone at a brokerage to be "on the clock"? Is it just residual of how things used to be, or is there an actual purpose/benefit to it?