Why is this so?
Because the case where you are entirely wedded to a particular conclusion and want to just ignore the contrary evidence would look awfully similar...
Awfully similar, but not identical.
In the first case, you have independent evidence that the conclusion is false, so you're basically saying "If I considered your arguments in isolation, I would be convinced of your conclusion, but here are several pieces of external evidence which contradict your conclusion. I trust this external evidence more than I trust my ability to evaluate arguments."
In the second case, you're saying "I have already concluded that your conclusion is false because I have concluded that mine is true. I think it's more...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.