For people who work in public administration: what actually makes weekend counter hours hard to staff?

I'm a regular 9-5 office worker, who enjoys bсg in my spare time, not in government, so this is a genuine question for people who actually know how these offices run. Last week I used three hours of PTO to go to a county office for something that took maybe 12 minutes once I got to the counter. The office is open Monday through Friday, 8:30 to 4:30. Basically the exact same hours a lot of the public is at work.

My immediate dumb thought was, "Why not have some staff work Tuesday-Saturday and some Monday-Friday?" Then I realized I'm probably ignoring ten boring operational reasons.

Is the main problem budget? Union or collective bargaining agreements? Building security? Needing IT or back-office staff available at the same time? Or is weekend demand usually not high enough to justify it? Umm, actually I'm not saying that usual government workers are lazy thing. The employee who helped me was great.

I'm honestly curious what the limitation looks like from inside public administration, because from the public side the hours can be brutal.