I see a lot of good comments here, but the fact of the matter is that the novelty of editing has worn off for many of us. In the beginning, when Wikipedia was small, or when it was new, or whatever the reason, it was fun to keep an eye on a few pages. That novelty has worn off, and along with it, any desire to fix the little spelling errors I find along the way.
As a side note, Wikipedia had, at one time, a large number of articles about my profession. None of them was accurate, at least in the US sense of defining many terms, specializations, and equipment. Maybe other parts of the world call things differently, but I doubt to the degree that Wikipedia was wrong. Still, I wasn't about to go re-write and fix links in every article -- even if I would have been able to find sources.
So frankly, I've given up. Yes, I notice spelling errors on Wikipedia. I just read past them. It's not worth fighting with people over and it's not worth my time to fix. My interests lie elsewhere. Sorry, universe.
Also, misspellings and bad grammar on the internet are cool. Just look at some of drivel published by actual legitimate news sources (AP, I'm looking at you. Would it kill you to spell check an article before posting? I know it wouldn't fix the "their/there" and related problems, but it's a start.)