Everyone's already said why (a long time ago too), but the problem is so frustrating that I'm compelled to say how much it irritates me too! I'm still finding mistakes in stories from years ago, even if they'd been proofread by others and reviewed since.
Until a year ago a friend and I were each other's proofreader — editors really, since we'd trade detailed notes and take some ownership. Sadly, she was kidnapped by real life and I haven't found anyone else. (Those who've offered to take over for her either don't know about the "adult" aspect of my writing or don't know about proper punctuation. Yes, I'm living dangerously now.) Even having a proofreader isn't perfect because after proofreading someone else's work twice it becomes harder to catch that author's mistakes. Like with our own stories, our brains tell us, "Yeah yeah yeah I saw that already!"
Whether or not I have a proofreader, when I believe a piece is ready to post I just walk away. Like Daye mentioned, for at least a few days (usually a week) I don't look at or think about it. Only then can i see most of the glaring mistakes, though still too few of the smaller ones. If it looks like my previous pass sucked then I'll finish and wait again. Eventually I would send it for proofreading and, yet again, not even look at it until she returned the marked-up copy. (People changing their stories while I'm proofreading them is my biggest pet peeve.)
But crimminy! One time I re-read and fixed a story that I hadn't looked at for over a year; then two years after that I read it again and found more real mistakes. (I'm not counting things I would have done differently or better.)
_C_, you're hardcore! A printer (with a scanner too) is high on my list of things to lust after, but I simply don't have space for even a small one. On the other hand, the last time I tried printing a markup copy of something at the library their print server refused to release the job for ten minutes, so maybe the terror factor will push me over the edge to get one.
brohne, I may give "backwards reading" a try for finding typos, but if I keep seeing "Worship Satan!" then I'll have to give it up.