If the child is not speaking yet, all they can hear is sounds and tones of the voice. They cannot make sense of the words, so they take comfort in the soft playful sounds. As they start to develop and learn words, then they may start to have an opinion on the content and often will repeat the words and start to use them and test them out. That's why I personally don't like 'choo choo' and the like, as it just means the child then has to learn that choo choo means train, when they could just learn the one word and meaning.
Basically you are expecting way too much of little people and as long as they are happy and the person giving them attention is being kind, who gives a stuff what comes out of their mouth? They could be swearing their heads off, but if they say it in a cute voice way, then the baby will still be happy.