Because it’s hard to concentrate on anything. Not because you’re sleep deprived, or because all of your brain cells have been sucked down to make breast milk, although these are obviously true too. It’s because, unless you’re physically away from your baby and someone else is watching him/her, you can’t really focus on anything else. Not deeply. You can’t do the kind of work that requires you to lose yourself in it. Because if you lose yourself you might forget to feed your baby. Or to move him away from the window when the sun blares in. Or you might not hear a sound that means something’s wrong.
OK, probably your baby is not going to let you forget to feed him/her. But the other stuff is real and could happen, and you’ve always got to be available, listening and watching out of one corner of your mind. And that means you really can’t do the deep focus kind of work. If that’s going to get done, you need time when someone else is watching and listening for you.