Well, the thing is, you have a few factors at work here. First off, Dan and I (and Carson) are amateurs. I had a technical writing course 20 years ago in college, but I can't remember anything about it except how to make transparencies for an overhead projector (which I guess aren't even used any more).
Then you have the time factor. Unlike a lot of professional sites, we don't get advance review copies of games (WTF?! is the only exception so far), so we are sort of under a self imposed pressure to get the review out as quick as possible, because the first week after release is so important to the people buying the game. Because next week, they'll have new options.
Playing a game and just then reviewing it isn't easy in a week. Much less having perfect copy editing.
I used to run into this in RPG reviews (which I used to do, being a geek). A lot of publishers would complain about reviews, saying they aren't professional or up to NYTimes quality or weren't playtested enough. Well, yeah, pay the reviewer hundreds of dollars, give them several months to write it, and yes, you should then expect a professional review. But if not, well, expect less than perfection.
Lastly, I would also point out that quite a few games are themselves riddled with spelling mistakes (not to mention bugs). If games that are worked on for months, and have gone through all sorts of testing and approval processes still have a lot of that, well...
Still, the biggest thing is time. If publishers/devs want better reviews, send us advance copies. It's really as simple as that.