@croydon - Unfortunately, it's rarely as simple as that. You're assuming that the parents have the vocabulary to pass on to their child, which might not be the case, especially for immigrants. You're also assuming they have the time to spend with their child, talking and reading to them so that they will learn new words.
In fact, if parents have to work more than one job the kids just simply hear fewer words at home and end up with a smaller vocabulary than their peers.
Teachers might know the importance of teaching vocabulary, but if one child can barely count to three and another child in the same class has a large vocabulary, then it's going to be difficult to teach both of them at the same time. It will be impossible to catch the first one up to the second without neglecting the second.
A good vocabulary is so important, but it isn't a simple matter to ensure that every child has access to one.