[petercooper]

And this is opposed to the threat of having mail that needs to be signed-for turn up at your house only on a weekday while you are at work.

Surely this is a universal and solved problem though? Most couriers (e.g. for most larger/heavier items you'd buy online) will not deliver on Saturday unless you pay quite a bit extra, so weekday-only delivery is already an institution of sorts.

That aside, maybe your issue is, secretly, USPS's primary motivation. If they don't have a Saturday service at all, people like you can't request Saturday delivery and will need to go pick up your packages from the depot. That'll save them a lot of time and money.