ebook etiquette

I've been checking out ebooks from the library. A friend gave me her old Kindle about a month ago, and it has been wonderful because with a newborn I haven't been up for weekly walks to the library to check out 5 books at a time. Now that she's had her 2-month vaccinations, we're starting to take her out more. We haven't been to the library yet, but I imagine it will happen at some point. So the ebook loans have been great. They have the same 3-week loan period as regular books. It does not take me 3 weeks to read a book, even at my slowest (hence the 5 books every week routine of summers past), so I return books when I'm done for the next person on the list. Apparently a lot of people don't. My friend just lets hers expire, and since I've been the first person on the wait list for a couple of books for about 3 weeks now, I'm guessing she's not alone. I don't get this. It seems like good etiquette to return ebooks when you finish with them instead of letting them expire. It's not like you need to find time to drive to the library to return them. It literally takes seconds. I can see not remembering to do so, but for a few days, not a few weeks. Or do I really just read faster than everyone else?