Put yourself in the kid's place: You don't know how to calm yourself, you don't know how to keep yourself warm, you don't know how to get around, you don't know how to get food. You are 100% dependent on people around you. The only thing keeping you alive is the goodwill of those people, and your ability to alert them to your problems. The default is no assumptions, just scream when there's a problem - assumptions are learned.

I was just watching some baby birds who kept calling for the parents for food, and it must be ancient evolutionary speaking as well?

Evolution's indifference is sometimes indistinguishable from cruelty. Presumably we're all descendants of the human and pre-human babies that freaked out at the flimsiest sign of abandonment. Presuming further, the ones who didn't may have had better outcomes for themselves and their parents 99.9% of the time, until e.g. they got lost in the tall Savannah grass just one time, and because of that never got to grow up and have children of their own.

Even if it only happened to, say, 20% of the quiet babies, it's enough to surpass the costs of paranoia.

I assume it is more of the "if the parent leaves me, I am going to die" reflex that is ingrained in babies, so the danger is great enough to warrant panic, even though it is improbable. Plus, very young kids don't have object permanence.

I heard of a parent telling a young kid that beyond the turn on the road there is a Foobarator, and kid dutifully keeps within safe distance without knowing what's a Foobarator nor what threat it poses.

I remember at age 7 being afraid of tigers and dinosaurs at night, sometimes I watched tall coconut trees. I also asked my mum if she will abandon me for a mistake.

Infants especially sleep BETTER when there is noise and people around. For an adult noise means something is possibly coming to harm you and your family at night so it awakens you. For an infant, lack of noise means abandonment and the response is to cry as infants are helpless without their parents

Oh interesting, so having ambient noise helps babies sleep?

I had an extremely fussy colicky infant. I never thought that noises would help him sleep. But I noticed he would sleep if I was holding him and talking to someone. One day he was overtired and screaming because he could not sleep, and I turned on the vacuum. He went straight to sleep. It got me thinking about why that would be. Almost all adults would not welcome a vacuum on to fall alseep. We got a white noise generator after and it helped him out.

Like managing a team of diverse thinkers: "Do what works for you(r team)!"

I've read somewhere that white noise is similar to what they hear while they are inside their Mom, so this puts them into a familiar environment, which makes them calm.

They make stuffed animals that play white noise or even human heart sounds. Not sure whether there is scientific data on this, but a lot of parents use them.

I suspect that there were evolutionary pressures at play here.