You have real world knowledge of what makes entertaining TV, and perhaps how this particular show actually operated. And you are correct. In the real world, Monty Hall always revealed the worst of the two prizes that the contestant did not select. In the real world, you can use common sense and assume that Monty Hall will use that algorithm.

But in a mathematical probability problem, you can't do that. For example, if there's a slightly smaller object and a slightly larger object concealed in two different sized boxes, common sense says that a person would put the larger object in the larger box. But if it's a abstract mathematical probability problem, and you have no information on the algorithm used, then you have to assume the concealer flipped a coin when they decided which box to use.

If I'm the one setting up the big-small game, and I use the random algorithm, the smaller object is going to be in the bigger box about half the time. If my mom is the one setting up the game, the smaller object is going to be in the smaller box every time.

In abstract mathematical probability problems, if you don't know what algorithm was used, you aren't allowed to bring human behavior in, unless it's stated in the problem.