Because it is a human drive, and for many in the US anyway, working a lot is the only way to get a decently comfortable life - if it gives you this at all. For a decent amount of folks, they just work jobs that keep them from children (nursing and retail work, for example). We could change this a lot of places with worker protection laws. I don't have children, and support more robust support for parents and things like maximum work hours and the like to make sure folks can raise their children, without the poverty that some folks would have when working less.
To complicate it further, birth control isn't made free for everyone and in many areas on earth, sex education is poor if it exists at all - not to mention that abortion services, if they are legal where you are, aren't always available either - for those times when birth control fails or other complications arise.
I'll also say that most of the "we have too many people on the planet" arose from attitudes looking down on others, often from countries of origin, racism, and the like. We know how to help lower birth rates (widely available contraception and sex education) - but we aren't doing this - and we can feed everyone now, but we (as humans on average) don't put it as a priority - as seen by not shipping surplus to folks that would happily eat it.