How do you know that Catholics worship the statues? What makes a graven image is whether or not one worships the object instead of God himself.
You are quite right that the statues are not in and of themselves required, and the Catholic church does not require them. Some choose to erect memorials to the Saints in their honour. Some do not.
And those that do may be committing idolatry. If they venerate anyone or anything othr than God— idolatry. Mariolatry just happens to be one of the subjects here.
Hoss