Movie theatres make almost nothing from ticket sales. They have to pay a huge lump sum (upwards of $100,000) up front just to be able to get the movie and show it for a month. Often they simply lose money on it! So the crazy price of snacks is an attempt to recoup their investment faster and hope to get some profit.
The other model is for the theatre to simply pay 95% of ticket sales to the movie studio for the first week (and a bit less as the weeks go on). This essentially guarantees the theatre loses money on the film (due to all the other overhead that easily eat up that 5%) but it’s less risky if the film is a failure. Either way, they only make money on food!