It's no use trying to answer the question because you have no choice in being born into the world. You just become. In the scheme of things it seems that we transmit a new form of human each tome we mix our genes. So, while we die, new humans replace old ones.It's called evolution.Why that happens is something we can only speculate on biologically. Maybe new forms of life that will eventually be able to see whether other life exists elsewhere in the cosmos. And then who knows? And still why? The answer to why do we exist? To live your life! That's all. How you do it is just a matter of convenience to you. Good or bad, that's up to you. Goodness helps others live their time in the world easier and bad makes their lives all the harder. Either can lead to general progress or retrogression of humans as a whole.
In the long run, all we do is overpopulate the world, which will eventually right itself, by whatever means.
So, enjoy it if you will, as you only have one life. Whoever created the cosmos knows why it was made.