Same reason people appeal to moral absolutes in order to justify their own desires. "I want it because I want it" isn't good enough for us, which begs the question, why do we have such low opinions of ourselves?

Just throwing this out there -- might have something to do with the part where most of us identify with a fictitious separate self bound by a body that lives disconnected from a world it can actually rely on to meet all of its needs if it stopped believing in its own separateness. Which has something to do with chronic fear, which has something to do with the trauma of experiencing the self as separate. Chickens and eggs. The cause doesn't matter, the solution is the same: Ego death and/or trauma healing, call it what ye well.