I don’t know which country you’re in (and don’t disagree with you) but even if the estimate of 30 minutes to shipping labels were accurate, that would still be a net win where I am in Texas - the line at the post office is regularly longer than that.

Because staffing can/has be/been reduced since they made it possible for people to print their own labels. They aren't interested in making the queues faster.

You still have to wait in line even with the shipping label because they have to scan it into the system.