It's true that we OFTEN backshift in reported speech, but not always.
When it's a "universal truth" (something that is, was, and always will be true [unless proven differently by scientists of the future]) we often keep the present.
Examples: The Earth is round. The penguins is a bird that can't fly.
You can also use it when you want to emphasize that something is still true, especially if the listener may think it's no longer true if you backshift to the past: He said you are pretty. (You don't want the listener to think that "he" said you "were" pretty years ago but you're ugly now, for example.)
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