This is something good about media, they (plural of medium) have a discrete length. That is, not just any series comes to an end, or fails to come to an end as the case might be, but each piece, each packet, is of a discrete length. For instance, a DVD only has the episodes on it, a book only has the number of pages within it, a movie is only as long as it is.
A series might comprise several DVDs, and each DVD might comprise of several episodes, and each episode has a distinct length. But what is important I think is the DVD. A lot of it comes to how much effort people are willing to put into things, but also because people want to have given something “a fair shot.”