There's another reason that this is done. It's to separate the different copies of the characters. There are multiple of all of them in every park, but by keeping the sight lines contained like this there can be 9 Mickey's all over and you can never see two at the same time.
I took my kids to Disney last year for the first time. We went to go see Anna and Elsa. The line moved fairly quickly, and the group ahead of us went in through a closed door. Almost immediately the door opened again and we were pulled through. Rather than seeing the princesses, we were in a hallway of the house with ~8 doors, and immediately shuffled into one of them where the princesses now were. On the way through, I tongue-in-cheek asked the attendant what was behind the other doors. "Broom closets."
Can confirm, there’s only one Mickey, they time their exits and entrances and have a park coordinator (mostly women under the Mickey costume, by the way, because of costume size categories).
