Because it is actually your information, and thus no new information is obtained.
Hypnosis is very useful for giving out information because people are very suggestible during hypnosis, but almost worthless as a tool for getting unbiased information for the same reason. I don't understand why anyone would assume that a state of mind where someone is more suggestible would make them more likely to tell the truth, since environmental factors are what make us lie, including to ourselves, in the first place.