Because the kids are soft and weak.
No, previous generations ruined the world so much that its no longer worth it to reproduce. Why would I reproduce? I go through all the trouble to find a woman, years of pursuit, I pay hundreds of thousands of dollars to raise a kid, all while being scared constantly of all the dangers the kid might face in the world, such as drunk drivers and bullies, and then when that kid grows up, in best case kid forgets I ever existed, and in worse cases, blames me for being too strict or not being too strict, for raising him in wrong religion or for not focusing on religion or for forcing religion, or for letting him choose, or for not letting him choose, or for not supporting him that one time, or for supporting him too much, for not giving him all he wants, or for spoiling him by giving too much, for sending him to wrong school, for not forcing him to study or for forcing him to study too much. And this is still not the worst possible. Imagine kid born with disorder.
I dont need to fuck myself over by doing something stupid such as reproducing. All previous generations made that mistake and they fucked us all.