One of the things that you need human staff for is simple safety in the event of an emergency, assistance for the disabled/elderly, and to basically provide some level of minimal physical security and and assistance when the automation goes wrong.
I suspect no insurer or council H&S person would sign off on a completely unmanned cinema as the risks to the public are far too high, and the risks to the premises and the expensive equipment is high.
At our local Odeon they combine the tick and snacks counters, but those same staff are also there to assist in things like dealing with failures of the automated machines, and as some level of deterrence from trouble makers.
Staff costs for a cinema are going to be relatively low, 4-6 staff is not a massive amount compared to the costs of energy and the rights to show the films, and one of the big failings of Cineworld going back many years was always (in my experience) they simply didn't have the staff on site to do simple things, for example we'd go to watch a new release, loads of people buying snacks but no hot dogs because IIRC only certain staff (more senior) were allowed to deal with them, so for a showing at peak time with hundreds of people no hot dog sales. Odeon on the other hand always had them available, or cooking as they made sure that they had enough staff who were trained for them.