It’s for a very simple reason, it’s because REM deprivation doesn’t result in death. 
This idea came from Pr Michel Jouvet’s first REM sleep experiments (on cats I believe) in the 60’s. This theory has been proven false and some people who have brain problems or who are submitted to certain medication don’t experience REM sleep (it’s the case with some barbiturates and anti-depressants). Yet it seems that REM sleep deprivation may cause learning problems but this point is not perfectly clarified.
kudos on the logic.
i can see how this theory could be exersized to multiple uses. What if I wanted to conquer a fear, or overcome/expand on a feeling? You could openly train yourself to taylor any need you have. If you’re scared of flying and you’re going on a long plane ride, lucid dream about flying and assure yourself of the safteyness. If this theory is true, then you would slowly erode the fear of flying. Hooray! I’m going to try testing this theory next time I establish lucidity. I’ll post back if I have results.