Most likely because when you die or something really tramatic happens there is a chemical in your brain called DMT the most powerful hallucinogen known to mankind that is released probably as some kind of defense mechanism the body is truly an amazing thing and why i say this is because i have had near death experience when i was a teenager because i drank ALOT and one night my destructive self mixed that alcohol with valium and oxycontin yeah i know extremely stupid but what happened is well i lost consciousness everything went black then i found me from a birds eye view looking at myself extremely interesting experience there is more to it than that but i don't feel like typing that much so basically i had your typical experience and a couple years down the road i had come across DMT and i did it and it was a extremely profound experience almost exactly like my OBE and i even saw light beings and "GOD" or what ever this incredible all knowing intellegent light it was life changing so after that i did research on DMT to find out that your brain naturally produces it. Now am i saying all NDE's are DMT trips no because how could i honestly say for sure it's just the experiences were so similar it's amazing but that's just one theory because the ones that have stayed dead obviously aren't here to tell us about it or are they?