This may surprise many of you, maybe even all of you, but outsourcing concessions at one time was the norm.
Most theatres used one of several national or large regional concession companies. Berlo Vending Co. out of Philadephia was one of, if not the, largest in the country. In addition to theatres they also operated concession stands in sports stadiums, race tracks, airports and amusement parks. Most of the theatres in the Philadelphia release district used Berlo, but others used Western Automatic Vending out of Buffalo, NY or Theatre Confections out of Rochester, NY.
All the major circuits that operated in our area such as Stanley Warner, William Goldman, Fabian, and Boyd all used Berlo. All the theatres that I attended or worked in during my youth used Berlo. Some of the independents such as my Roxy theatre used Theatre Confections. When I took over the theatre in 1970, I immediately replaced them with my own stand, a used Berlo stand. They were not happy that I wouldn't keep them, so they refused to sell me their stand and equipment. A few years later in the fall of 1972 I replaced that stand with another larger used Berlo stand. In the fall of 1987 I had a completely new stand built to my own plans more in keeping with the architecture of the theatre, but still until this day use the old Berlo pop corn and butter machines.
When Berlo was left go by a theatre management they weren't interested in taking the equipment with them if they didn't have to, so it was usually available for purchase at very low cost. I bought out Berlow equipment at several theatres that I took over for no more then $500.00 each.
As each of those circuits I mentioned were bought out by others... Stanley Warner locally by SamEric which was later bought out by United Artist who in turn were bought out by Regal, William Goldman who was acquired by Budco who in turn was purchased by AMC, Fabian by United Artist, and then Regal (Boyd went out of business), they all were converted to inhouse concession operations by the newer managements.
There is no one that I know of that outsources concession operations in this area in this day and age. I wouldn't even know of what company would be available to do it. It certainly doesn't make any sense as others here have already stated.
Running your own concession operations is not difficult, and is much more profitable when done inhouse, and certainly gives you the control that is important to your business.
What made it attractive in the past and maybe still does in other areas, is that the concession companies would finance new theatre builds for the exhibitor. Still not a good enough reason for me.