To represent Animal perspective.
The Tom and Jerry cartoon is about animals, they have to show their perspective. Humans are larger to them, they can't even see their full body together. So it's kept that way. Humans are not even a regular cast, so why to bother to make their faces when it isn't needed. It also helps to make the audience focus on Tom and Jerry only.
But later they included human faces maybe for fan requests or for unknown reasons. If I am not wrong then in the earlier episodes human roles are very few to negligible but later some humans become partially regular.