In all but the most niche cases, they do in fact know that you had a kid.
How would the IRS know that? The only way I could think of would be the Social Security department sharing the information with the IRS; and are they legally allowed to do that? But let’s even say that’s true; if the parents aren’t married and filing jointly, who gets to claim the child as a dependent? That’s a decision made by the parents (or local courts in case of custody battles), so not something the IRS would decide.
Basically what it seems to boil down to is that filing taxes is complicated because the tax law is complicated.
That’s my thinking.
Every large organization, private or public, that I’ve interacted has been basically just a bunch of different people in many different silos. I’m surprised to see so many people have this “well oiled machine” perspective of the government where apparently it is all seeing and all knowing.