That's because, unnoticed by their parents, tiny babies quietly learn grammar and syntax. Vocabulary comes along later, to match the skills that they have been picking up from birth.
Talk to your baby!
The interesting thing that I found when I came to learn Italian - in Italy, without the benefit of any formal lessons - was that the similarities between French and Italian vocabulary (yet with significant differences) were such that speaking French became much more difficult. I can still read...