Humans live in social groups. Humans are also great at fighting. On top of this, humans (like other animals) have an instinct to protect and care for their young. So, given this level of protection, if a baby has a need (food, hot/cold, needs care) it is much better for a baby to loudly communicate this to nearby adults so it survives infancy, because the adults can almost always handle external threats that might arise.

It’s been found that babies that are neglected cry less because they learn that it no longer gains attention from caregivers, thus also implying that cries are sometimes literally for attention and nothing more as well, from what I have heard. Basically, scare the mother into attending to the babies needs or risk alerting a predator that will eat both the mother and the baby.

There are individual differences between babies. Not all cry loud. In addition to the mentioned facts, the nutrition that the mother had during pregnancy also plays a crucial role. For instance are babies who were exposed to Fastfood significantly more aggressive.