Either because people hold delusional beliefs about the dreadful reality of existing here or they are just lying to themselves and don't wish to accept that existing gets worse as it makes them uncomfortable.
But I literally cannot take anyone seriously who says things like "life gets better!", they just come across as so insane to me as after all we are just slowly decaying from age and it's true that once people get older they will just decline, like if people live long enough they will just be trapped in nursing homes being tortured by old age, I'm so disturbed by old age which is of course a reason as to why I always wish for non-existence.
I cannot stand those who glorify life as I just know that as time goes on I will become even more tired and want to die even more, existence is such a futile burden and it's terrifying how there's unlimited potential to suffer endlessly in this world where chance so cruelly determines everything, death really is the only relief for me.