Because real world information has sub-second resolution and a healthy market should reflect that. It’s a continuous auction. Some markets are open longer such as FX.
The NYSE is closed between 16:00 and 9:30, plus all weekends and bank holidays. AFAICT real world information doesn't stop in the closed hours. So how is it credible that it's super valuable that trades can happen during the open hours at sub-second resolution, but we're suddenly ok at 16:00 with a 17.5 hour resolution?
You are begging the question. Why should a market reflect that? Why is that preferable to a daily auction (throw in a stochastic cut-off time to thwart HFT even more)?
Because you can’t make sure that all auctions happen at the same time. Consequently, news will disproportionately affect stocks having an earlier auction. That’s just one reason I’m mentioning.
