harkm is one of only two people who answered your question. The settlement rules are such because you can deposit money later than when you make the trades, even today. You have 3 trading days to deposit money into your account. This is useful because bank clearing still can take that long. Given this is the case, the funds really aren't known to be available to your counterparty until the settlement date.
This tread is outstanding. The OP was right on the "money" and the folks that responded missed the point. Except harkm and ruminate. There are two brokerages that I'm aware of which provide a means to avoid the settlement requirement.
%% What can go wrong HaMow??Plenty; study annnual report of single stock or ETFs.Plenty can go wrong.................................................... I enjoyed Fidelity's ContraFund annual report this year; actually amazing all the things that can go wrong. Since they make money on commssions, AUM , get used to it. Thanks for the question