Because there’s a ton of research that we adapted to do it for good reasons:
Infants between 6 and 8 months of age displayed a robust and distinct preference for speech with resonances specifying a vocal tract that is similar in size and length to their own. This finding, together with data indicating that this preference is not present in younger infants and appears to increase with age, suggests that nascent knowledge of the motor schema of the vocal tract may play a role in shaping this perceptual bias, lending support to current models of speech development.
Stanford psychologist Michael Frank and collaborators conducted the largest ever experimental study of baby talk and found that infants respond better to baby talk versus normal adult chatter.
Because there’s a ton of research that we adapted to do it for good reasons:
TL;DR: Top parents are actually harming their kids’ developmental process by being snobs about it.
You can also just talk normally to your kids without being a snob about anything.