I think **chriscya **hit it in one. Not all babies’ cries are loud, and not all of each baby’s cries are loud (if you see the distinction), but the ones that are are really annoying because they work.
Babies do do a whole lot of quiet whimpering, trilling, cooing and smiling to get their servant/parents to do their whim. When they do cry (barring colic, which no one can really explain) they have different cries of different pitches and irritant levels to communicate different things, all of which need pretty immediate attention: discomfort, hunger, boredom, sleepiness and pain. The loudest of these is the “pain” cry. This is the cry that, saber-toothed tiger bedamned, means the baby needs attention RIGHT NOW! And it’s so distressing for adults to hear that we cannot help but respond quickly, or suffer very real emotional and sometimes physical pain. If these needs are attended to, then the baby settles back down into quiet grousing, most of the time.
OTOH, I think you’ve perhaps made a erroneous assumption in your OP: a baby may have been *near *it’s mother at most times, but not always as near as you’d think. Prior to the invention of knots (as in, knotted fur, fabric or even vines or grasses to make a baby sling), a human mother would need to set the baby down to gather or prepare food, wipe after defecation, build a fire or any other of a number of basic survival tasks. Even with a mate to provide food for both of them, she’s got to do some things besides just lay there for 18 months holding an infant who can’t walk beside her. We face a very real dillemma, without fur for an infant to cling to and no pouch for our basically immobile offspring for a very long time. So even 10 feet away from her baby lying on the ground means the baby needs a louder form of communication than a babble.
OTOOH, once the sling was invented, crying was much, much less of a problem. Babies carried in slings with constant or near constant parental contact don’t cry very much. They spend most of their time in what’s called the “quiet alert” stage of consciousness. So some of what you experience with babies crying is because, once saber-toothed tigers were no longer an immanent threat, people developed all sorts of non-evolutionarily driven habits of baby raising, including separate beds, playpens, daycare and other convienences which parents find nice, but babies find distressing. So babies get bored more, and distressed because mommy isn’t near (they’re still wired to think the saber-toothed tiger will eat them if mommy’s not around to scare it away), and they cry more than they would if they were carried all the time. You’re not seeing a clear “in nature” picture of babies’ behavior any more.
So they need to cry if something’s really wrong, but don’t cry as much as you think if they’re attended to quickly. If there were once babies who literally cried all the time, rest assured they’ve been eaten and aren’t breeding.