I will say up front that I don't think the social good is worth what we are collectively paying for it, but I do think the market hours are a reasonable device. This is basically because there are humans involved and they need to sleep (Matt Levine has written about this).
If you want the best price, you need to have all of the market participants bidding together. Market hours serve as a coordinated period in which ~all market participants agree to be online and bidding.
Prices, thus, get stale overnight. But we assume that that is mostly okay, as business is normally conducted during business hours, and we assume that transactions can wait until the next day. ACH transfers take multiple days! (technically so do stocks, but that's mostly invisible to retail traders).
If you're a retail trader, I would caution you somewhat against trading after-hours; there is very little liquidity and it could cost you 100s of bps more.
If you want the best price, you need to have all of the market participants bidding together. Market hours serve as a coordinated period in which ~all market participants agree to be online and bidding. Prices, thus, get stale overnight. But we assume that that is mostly okay, as business is normally conducted during business hours, and we assume that transactions can wait until the next day. ACH transfers take multiple days! (technically so do stocks, but that's mostly invisible to retail traders).
If you're a retail trader, I would caution you somewhat against trading after-hours; there is very little liquidity and it could cost you 100s of bps more.