determiners - much - very much, many - very many?

In case of uncountables, the large amount expressed by much like in

much water, much money, much sand

can be increased by prepending very like in

very much water, very much money, very much sand

But in case of countables, the large amount expressed by many like in

many trees, many people, many children

can not be increased by prepending very? So, the following is wrong?

very many trees, very many people, very many children

Is there a comprehensible explanation for this difference?

And what is/are the correct expression(s) to increase many?