It’s not discrimination man. People (including bank and post office workers) work during 9-5 working hours, so it makes sense that these services are only open during working hours.
You're right. Why do their customers insist on working the same hours they do? You'd think they'd work different hours so they could run their errands when things are open.
And businesses like that could create their schedules such that all employees would have some time for errands during normal working hours, but they usually don’t because it’s easier not to.