Because so far humanity has yet to develop a means to have kids that isn’t through natural means and thus is the only way the species can survive. You can be childfree, as that’s a personal choice that shouldn’t be taken away from you but the same can be said of anyone who do and shouldn’t be told not to. Additionally, for alot of people its’ a biologically ingrained thing to want to have kids, usually after seeing kids out in the world and having a subtle thought of “I wonder what my own kids would be like”, I know I feel that and I’m a guy. When we have technological means to have alternatives to natural pregnancy (which by the way we have been making it safer as time goes on), then we can have discussions of the cost and benefits of natural and artifical births in earnest, but right now natural is the only thing we have to ensure the species survives.