Because for me, each scene of the preview that I see is kind of a spoiler in itself. Most of them are dialogue-driven and can sometimes give hints about what to expect next. I personally would like to know nothing about what comes next and be surprised the moment I actually watch the episode. So I just skip it.
But that's just me.
EDIT: Eek. I hope you all know it's "previeWs" for the topic. Guess I type too fast.
