There is the simple fact of instinct: the instinct to survive is tremendously strong. At least in the majority of people.
That's why, when someone is told they have terminal cancer and will take at least 6 months to slowly and painfully die, they don't step in front of a train. They fight to survive, and many of those "6 month prognosis" people will fight death and hang on for 12 months, despite the suffering.
No .. people are not about to go out and readily willingly kill themselves. Suicidal ideation is almost only that .. ideas. Not action.
Despite what some people here are saying, there are some qualified teachers of Buddhism who teach karma and rebirth as metaphors.
As for Buddha and the 1st Noble Truth: "Suffering occurs". That actually is not pessimistic. It is only in our Western culture, where we are supposed to be in denial about unpleasant realities, that it SOUNDS pessimistic. Because we have no coping skills for sorrow or suffering, we brush it aside as quickly as we can and refuse to look at it. We think that if we can just keep busy enough, just pleasure ourselves enough, just distract ourselves enough .. then we will avoid our inner discontent. It doesn't work like that, and all the frantic screw-ups of the rich and famous, including their inability to hold onto relationships and the drug overdoses, should be a clear indication of just how our frantic drive to run from reality actually doesn't work.
In Buddhism, the world "suffering", is perhaps an unfortunate translation. Because what is referred to is everything from that mild discontent that we all experience, through boredom, through unstable emotions (happy one minute, annoyed the next), through loss (no matter what we have, nothing is permanent) .. through to what we think of when we think of the word "suffering" (physical or psychological agony).
And the thing is this ... WE cause our own suffering. And we don't have to. And this is what the Four Noble Truths are about.
What you are proposing is like saying: "Got a headache because you keep on hitting your head on the wall? Why not just blow your brains out?" While Buddhism says simply "stop hitting your head on the wall".
There is nothing negative about pointing out that you are hitting your head on the wall and this gives you a headache, and there is nothing negative about saying "You can stop doing that".
As for karma and rebirth. They are concepts. Buddhism teaches concepts, but the primary and the important teachings are the techniques. The techniques of meditation are central, but there are other techniques too.
These techniques "do" things ... they get you to SEE the wall, to see that you are hitting your head on the wall, and to see the connection between hitting your head and getting a headache. And they retrain you so that you no longer compulsively, automatically hit your head on the wall. And they teach you to pay attention to other things in life besides that stupid wall.
One does not need to use the concept of karma or rebirth to use the techniques, nor to get results from the techniques.
And that's the beauty of it. It works.