Why?

When we talk about ingredients of some kind of dishes: why do we say lentils not lentil; beans not bean; nuts not nut, although we say rice not rices?

I believe we say lentils not lentil because we don't mean one seed of lentils or one bean, but we mean a quantity of them both so we use the plural form, whereas with rice the word itself means a plural from although it is uncountable.

Thanks in advance.