To answer this simply, it's because I am overconfident in my ability to write spectacular stuff.
Really. I have a very difficult time taking my first draft and changing anything. Not word choice, or sentences, or content.
This was most evident in high school when I didn't even read over an essay I wrote, but would simply print it out and hand it in. I got decent grades so I never felt the need to edit it. I felt no need to make it better.
I even thought it couldn't get better. I was silly.
Now I realize it can get better, I just don't exactly want to work on it. It certainly takes more energy for me to edit what I have already written then actually write it. So, that has always translated into not putting that extra effort into it, and getting what I got for what I write at first.
Of course, I realize that is no way to become professional. I need to scrutinize every word choice, every sentence. It becomes imperative that what I write is not only the best I can write, but the best I can edit. I wish I was there already, able to give a great list of 5 easy steps to becoming the best editor ever, but I don't. I don't think one really exists. It has to be what works for you, what you can do to produce the best edited material.
One thing I do to help is to read blogs and books by other learned writers. Gleaning as much advice as I can is one way I have become more aware of my need for editing and being able to actually do it.
There are so many books and blogs that you can't really go wrong by just looking and going with the first thing you find. Of course, if it isn't helpful, stop reading it and move on. But everyone has something to tell about their experience and you can never have too much advice.
Now, if you will excuse me, I need to check on KM Weilands' blog and see what's new! She is my favorite blogger with real advice for writers!