Markets have chosen to concentrate trading to specific hours so as to concentrate liquidity and to help out the professional traders. If everyone's trading 24/7 then it means you need to be prepared to trade 24/7. Having defined market hours means defined hours in which important things can happen. And since the people who work in the market can also define the hours, they've chosen to give themselves reasonable working hours.
In a world where the major markets are trading 24/7, traders need to be at their desk 24/7.
> Why can't I move money from one bank account to another on Sunday?
Instant money transfer means instantly draining someone's bank account with no recourse. Checks have plenty of opportunities to stop the money, so you can write a check on a Sunday as easily as a Wednesday. Wires are more irreversible and so you can only send those when the bank is open and there's someone to double check any suspicious behavior.