Because to think otherwise is, at best, socially frowned upon, the vast majority of us insist on believing in our "purpose". We then create a reinforcing delusion that some grand cosmic architect with perfect knowledge and foresight placed us on this terrestrial chessboard and said "Here - this is what I have in mind for you" - and then proceeds to invisibly guide our journeys.
The truth is that in very large degree (approaching totality) our existences are in the deepest sense of the word "meaning-less". And for the most part devoid of true joy. We may on occasion experience something we label as "happiness", but my strong suspicion is that this is nothing more than tiny deviations from an at best neutral baseline. And in the end, all but the tiniest sliver of us will be well and truly forgotten - lost to history for all time - within an eye blink of our passing.
As Lincoln famously and in retrospect ironically noted in his most famous speech - "History will little note nor long remember the words spoken here today."
This is in fact our fate.
Yet we are systematically conditioned from birth to disbelieve this fundamental reality. Perhaps there's a societal assessment that this is an unpleasantness not to be reinforced in genteel circles. Perhaps there is a widespread concern that some/many/most of us are not strong enough to accept this less than appealing prognostication of the arc our our brief period of animation. Or maybe it seems inconveivable that an impossibly unlikely confluence of events over billions of years would give rise to our existence, only for it to mean ... nothing.
And yet, I submit, this is the inescapable incontrovertible reality.
Consider this fact: according to the latest astronomical observations, our observable universe (which may be only one of an unknowable number of universes) contains 7 septillion stars. This translates into one trillion stars for each and every human being on the face of the earth.
If the existence of our life-sustaining planet is the proverbial "billion to one shot", this means there are one thousand such planets ... for each and everyone one of us.
Now imagine if you became the greatest person in the history of our world (choose your own definition of "greatness" - kindest, richest, most powerful, most spiritual - whatever you best relate to) - and then imagine that with that gargantuan achievement ... there are a trillion other lives in our universe that can make the same claim.
We are truly indistiguishible in this regard from the gnat we swat or the bacterium we kill with our antiseptic mouthwash. We are absolutely making a distinction without a difference to think of ourselves as on a different level that they.