I can't really explain why I don't do it. I guess because, most of the time, my summary would just be a pull quote and I figure people can just read the story to get those lines. Or because, when I summarise my fiction it always sounds faintly ridiculous, or gives the entire game away.
Take my latest piece, which essentially boils down to "Sam and Gene talk about Sam's teenaged guitar playing just before Sam dies, so four years later Gene goes searching for teenaged guitar playing Sam." Well, okay. Now that I know what it's about, why would I want to read it?
But, you see, other people aren't like me. They apparently want to know what they're reading before they get into it. They want some sort of contract with the writer. "This is what you said you were offering, I can now decide whether that is true or not." If I had done the pull quote thing in summary to with a circus mind that's running wild, it still wouldn't have given an accurate view of what was to come.
And maybe that's what I don't especially like. I don't want people reading my fiction based on false assumptions of what they're going to get. I want them to be surprised, without having to go back on that unwritten contract. Because I, quite often, like to do things like unreliable narration and subtext and twists in the tail. I like to be unpredictable (though, not completely, obviously.) And I don't especially like being thoroughly manipulative in order to do so.