ELI5: Why can I "hear" perfect pitch in my mind, but when I try to sing I sound like a walrus giving birth to farm equipment?

When I say "in my mind" I'm referring to my thoughts/imagination. I'm not talking about hearing myself when I'm actually singing. For example, I can think/imagine singing a song with perfect pitch, yet if I were to try and actually sing it, I would sound horrible.

UPDATE #1 Several people are comparing this to imagining something visual, such as a beautiful painting but not being able to paint it. However, is this the same as imagining audio?

UPDATE #2 Apparently a lot of people really enjoyed my walrus analogy. Then someone suggested I start a subreddit called /r/stanisms. So I did, and it exploded! I'm having so much fun with this - thanks for making my night, Reddit! :D And thanks to the person that gave me gold!!