Aside from people needing to sleep (back when computers didn't do our trading for us), there's also the matter of information flow: the reason quarterly earnings announcements happen after market close is to avoid giving slightly-faster people an edge.

If you could read and digest an earnings announcement faster than all your peers, you could make trades based on the new information before the price has moved.

While yes, some types of trading does rely on exploiting (usually small) information asymmetries, and "incorrect" pricing, it's much fairer if people have time to digest big required disclosures and all be able to get their orders in at (essentially) the same time: the opening bell the next day.