Because "read of things" means "read (that) of things" while "read things" can only happen when the "things" are present (that is, we cannot read things that lived five thousand years earlier; we can only read something about the "things"?).
We can read of things that happened 5,000 years ago in the Near East, where people first learned to write. But there are some parts of the world where even now people cannot write. The only way that they can preserve their history is to recount it as sagas--legends handed down from one generation of story-tellers to another. These legends are useful because they can tell us something about migrations of people who lived long ago, but none could write down what they did. Anthropologists wondered where the remote ancestors of the Polynesian peoples now living in the Pacific Islands came from. The sagas of these people explain that some of them came from Indonesia about 2,000 years ago.
NOT A TEACHER
I'm not sure if I understand your question. In your sentence, you could use "of" instead of "about". 'We can read of/about things that happened 5,000 years ago.' "Reading of/about" means finding out information from some particular source (magazines, books, internet and so on). See also, read - Definition from Longman English Dictionary Online, definition #2.