Just because you can not be absolutely certain doesn't mean it's a coin toss with both options being equally reasonable. If someone wants to stop existing, they have the power to make that happen, while the converse is not true, thus choosing to assume someone wants to exist is safer. The overwhelming majority of people want to exist and the few who don't tend to be in situations where it easily could have been foreseen that life would be problematic; if you've lived a mostly happy life for a few decades, odds are pretty good that your offspring will do the same, thus statistically choosing to bring someone into the world is far more likely to be what they want.
But going further, the idea that death negates the benefits of life is absurd. If you read a book and are sad when it ends, that means it was a good book, and you are better off for having experienced it. The ending does not annihilate the story, it completes it. And if the story should have some bad writing in it, that may be undesireable, but all the good writing is still there to be enjoyed. No matter how bad life seems to get, the good moments still happened and they can never be taken away from you. Sure we'd all like to avoid unnecessary suffering and postpone death for as long as possible, but only because we would rather fill that time with the joys of life - if you had to choose between experiencing extreme pain periodically or spending the rest of your life in an inescapable coma, you'd certainly choose the former. Non-existance is not a pleasant alternative to life, it is a fate at least as bad as death, if there is any distinction at all.