If you prefer ice cream to cake when they are the only two alternatives, then why would you prefer cake to ice cream when a third, inferior, alternative is included?

It can be rational to do this. There's a paradox publicized by Martin Gardner demonstrating how. Unfortunately the best link I could easily find was a Reddit comment, but try https://www.reddit.com/r/fffffffuuuuuuuuuuuu/comments/gxwqe/why_i_hate_people/c1r5203 .