I think to preface first off, Morrison is often considered one of, if not the greatest American writers of the late 20th century—her work is very frequently held up as being as important to American literature as the examples you mention. I struggle to think of any of her contemporaries that are uniquely American that will be held up as much in the canon. The closest from her relative time period would be Salman Rushdie, but I would consider him to be an Indian Diaspora writer and not an American writer.

But there won’t be as many sources claiming it to be ‘the great American novel’ or ‘greatest American work.’ Part of this comes from recency. Beloved is still considered to be contemporary fiction—or at least was while I was in college and studying literature. It came out in 1987. Contemporary works are a bit harder to judge since we don’t know what staying power they’ll have. There just hasn’t been as much time as the examples you mentioned that came out in 1960 and 1925.