Why do brilliant novels so often fail at the finish?

A disappointing ending to a novel you've been enjoying has a peculiar power over us. At least it does for me. 

You can spend days immersed in a novel, recommend it to friends before you've even finished it, and carry the characters around in your head. Then you reach the final chapter and something doesn't quite land. The ending feels rushed, overly clever, relentlessly bleak, or simply at odds with everything that came before.

It raises an uncomfortable question. Can a bad ending ruin a great novel? I'm asking as I have read a slew of novels recently where this has been the experience.