What's the deal with only reading completed works?

EDIT: NOT HATING! I really am curious!

I've seen this here, in many other online areas, and even with actual books. Why do so many people sort by completed and only read completed?

In real life, my friend refuses to read multiple series (that she's literally bought the books for; it's not like she's waiting for a box set or something) just because the final book isn't out yet. I understand if it's one or two, but she's got five incomplete series that she won't read simply because she doesn't have every book, and then complains about not being able to read anything.

In a server, I rec'd a fanfic with over 200 chapters, over a million words, with every tag this girl wanted, but because it wasn't marked complete, she refused to read it and even told me I 'enjoyed the pain' of reading incomplete books. I've seen it on here a lot too.

So I gotta ask, why are people so against reading incomplete works, even ones that are constantly updating?