The simple answer is because of our human nature's survival instinct. Human beings will try to do anything to keep surviving, to keep living, no matter what.
In today's modern era, it usually means toxic positivity, optimism bias, & even for supposedly the 'smart' people, they usually fall into rationalizing away everything to sounds positive or hopeful, even when in reality it's not.
The saddest thing about life in my opinion is that there are winners, and there are losers unfortunately.
Some people are lucky, some people are not.
I even admit that yes, there ARE people who are truly, really happy living their lives, whether it's because they're successful, or have loving family, relationships, friendships, or doing something meaningful, purposeful, or even just simply plain content & grateful for this life ("life is a gift!").
But what about people like us here, for example? whom for each different various reasons, somehow just got tired of life?
There should be a painless, easy way out for people who want to exit this life, as we basically also can't choose when we're born in this world. But sadly, our human civilization is still not that advanced yet.