Why doesn't healthy = palatable?

I have never understood why it isn’t true that “good for you” = “tastes good.” It would encourage healthy eating, which would seem to make sense.

I know that many healthy foods do taste good, but the fact that we have so many “guilty pleasures” says something. Why don’t most people prefer spinach over corn?

I understand that it is a temptation or test in religious terms, but what is the scientific reason?