It is not so simple to make a fake diplomas becouse they do in special tipography. That is why people simply buy a fake diplomas. And sometimes the quality of fake diplomas is rather good that they looks like a real one. It is necessary to cheach diplomas by number but it is hard to do this.
Forgery of documents is a well-established business, the quality is not inferior from the original, only as a result of the verification, you can find out the truth.
But if the varification of diplomas is hard and nobody check the diplomas making fake diplomas0 becomes a very profitable business.