You can browse the Wikipedia article I linked. It offers a few possible answers to this why question.
As for myself I would simply summarize that making an important choice such as bringing life into the world, without considering the consequences, is already somewhat unethical. One should think before taking an action that has irreversible consequences to anyone. In this case, the person being born. I wouldn't say I'm an antinatalist, I just find it interesting. On a rational level I'm not sure there are many good arguments against some of the conclusions there. If there are I'd be interested in hearing them. The fact that you are personally grateful for your existence is a pretty weak argument imo. If you had never been born you would not be around to know the difference. However had your existence been different it is not so hard to imagine you might feel differently. Surely your life is not so peachy that a scan of that article will be incomprehensible to you. Then again, perhaps you simply lived a far better life than I.
I'm not taking a position one way or the other. As I said, I just find it interesting.
> However had your existence been different it is not so hard to imagine you might feel differently.
Considering how the vast majority of people don't actively seek to end their lives, I think it's reasonable to assume that they prefer to live.
The crown argument against these musings is that according to them the only way to realistically act ethically is to not have children at all - that is self-defeating and not sustainable.
Also it places emphasis on avoiding harm/suffering etc. Problem is, these are unavoidable parts of life and trying to minimise them at any cost is essentially attempting to not live.
I think the term sometimes used for such things is "death cult".
I did skim the Wikipedia article but it didn't seem to engage with anything I was thinking about.
Almost everybody's revealed preference is to stay alive rather than to not be alive, otherwise we'd see a lot more suicide.
> making an important choice such as bringing life into the world, without considering the consequences, is already somewhat unethical.
This is a view you can take, but it's not as obviously important as you seem to think.
An alternative view is "making an important choice such as FAILING to bring life into the world, given the opportunity to do so, without considering the consequences, is somewhat unethical."
If the standard was truly that you can't have kids unless you're sure they're going to have a great life, then we would have gone extinct millions of years ago.