The thing is, you can not chose once you are in the room. The fact that an [almost] perfect predictor exists implies that your choice must already be fixed at the point in time the predictor makes its prediction. Or the other way around, if you could still choose both options once you are in the room, say by basing your choice on some truly random and therefore unpredictable event like a radioactive decay - at least as far as we know truly unpredictable - then the predictor could not [almost] perfectly predict your choice, i.e. an [almost] perfect predictor can not exist.

So what you really want is to be in a state that will make you chose one box and you want to already be in that state at the time the predictor makes its predictions because the predictor will see this and place the million dollars into the second box. And as we have already said, you can not chose to take two boxes afterwards as that would contradict the existence of the predictor.