Why can't we use comma instead of semicolon?

You see, comma is used in sentences like this:

I met Rita, and she gave me a book.

And semi-colon is used in sentences like this:

I met Rita; she gave me a book.

To be precise, semi-colon is necessary only when there are two seperate sentences, and there is no article between them It is perfectly clear that they are two different sentences. It's just that the sentences are very much related.

Hope that helped.