Most nice brokerages will cover the float for the settlement as long as you don't try to withdraw the money outside of the brokerage. Also it's T+2 days now in

Most nice brokerages will cover the float for the settlement as long as you don't try to withdraw the money outside of the brokerage. Also it's T+2 days now in the USA at least. So this is mostly a non-issue in practice and has nothing to do with your actual complaint. ACH is T+2, Checks and everything else is T+2 to T+5(some paper checks can take 5 days to fully settle). This settlement, like the other commenters have mentioned is to fix any problems. I've seen giant companies screw up ACH payrolls, that they have to recall every ACH and re-submit. This T+2 is to fix those mistakes. Brokerages screw up sometimes too. T+2 let's them fix those mistakes. Yes automation and technology could make it all instant, but human error still happens, so T+2 is there to fix the human error, without having to reach out and undo transactions that have landed. No trades are guaranteed because you have to find someone willing to take the other side of the trade. This applies pre or post market. Some brokerages are happy to let you try and trade pre and post market, but that still doesn't mean you can find a buyer or seller to complete your trade, much less trades happen, so it's much harder to fill them. Large investment firms don't really get special treatment here, like you seem to think.