Stop us if you've experienced this before: the clock strikes 8pm and you realize that you really need to pick up your drycleaning. But you're right in the middle of your book! Well, you'll put the book down when you get to the end of this chapter, you reason.
30 minutes afterward, you can't resist taking a quick peek to see where you are in the chapter. That's when you discover: you're only a quarter of the way through.
If you've ever felt this sinking feeling before, you've already got a sense of why chapter word count should be so darn important to writers. Lengthy chapters can make readers want to give up on reading a story.