People are always uncertain what kind of world they would be bringing any potential children into, and as we look at the current era, with its boundless prosperity and opportunity unlike mankind has ever seen, people are just not certain that now is a good time to have kids.
Smartphones. The internet. Netflix. These are things kids would distract us from. In previous times, like during the ice age or the black plague, people didn't have anything better to do than have kids, but now with all the luxuries and technology we have, kids would get in the way of things like responding to people on Twitter. Do we really want to bring children into a world where I'm constantly distracted trying to check for updates on my phone? That's not fair to them. Or to the people I interact with online.
Also, with all that climate change and whatnot, people are predicting the world might not last another ten years. People have predicted that for decades, so one of these times it might come true. And if you think the world is going to end, the best thing you can do is stop having children. That's telling the world: "You can't fire us... we quit!"
And while we have in this world wealth and abundance like no time in history, there is one thing we have less of: responsibility. Apparently, kids require that, and it sounds awful.