Probably because people want to believe they are more than what meets the eye. We want, naturally, to believe that a part of us is somehow more than material. We don't want to believe that we are just a fragile structure of bones, organs, and muscles wrapped in skin When people look in the mirror, they don't want to believe that they're just a piece of flesh with an inevitable expiration date. One day you, I, and everyone else will cease to exist in any form whatsoever - however, some people out of either fear or arrogance, refuse to believe that. But as to why people believe it, well it boils down to human psychology. Fear and arrogance are the best explanations I can give.