Because I remember and it really kind of is. Even while you love it, and them; even when you manage to get your older child dressed and fed and played with; even when you're out walking or grocery shopping and keeping your life on track; it is the opposite of easy, and everything, from getting dressed to sleeping to eating, has the Newborn Tax of Added Effort and Fatigue. (Not to mention the mamas have been through the greatest physical challenge that most people ever undertake, and it ain't over after the baby's out.)
My heart goes out to parents. Give them some props and tell them how great they're doing, even if they have the baby over/under-dressed, or are/aren't cosleeping, breastfeeding, crying-it-out, or doing whatever you think they should be.