This is something I'm guilty of myself, and it's a strange behaviour.
We can rule out some of the obvious cases. Blogs that discuss mainly current events tend to date in much the same way as newspapers. I think there's a tendency for a lot of writers who don't have a particular dedicated subject to drift towards either 'discuss current events' or 'link to cool stuff someone else has posted', if for no other reason than that these provide a fairly reliable source of new subject matter.
But if the subject matter is more broad, old entries are probably just as interesting as new ones. Perhaps even more so, if you think that people use up their most interesting insights early on in a blog's life. If you switched the dates, it's not always clear that people would even know. This post, for instance, would have read the same if I wrote it last year or next year.
Some people like to comment, and take part in a discussion. That's a good reason, but those people are usually a small minority.
So what about the rest?
My guess, for what it's worth, is that people get used to a very particular process of reading. Clicking on a website is like pressing a button that says 'entertain me'. Sometimes it works, sometimes there's nothing there and you move on. RSS readers are even more extreme - there's the bold 'new' entries, and then there's 'everything else'.
Now, in theory you could just click on the archives and hunt around for other stuff, trying to filter out the bits that aren't relevant any more.
Then again, in theory you could also go read a book, or talk to your co-worker down the hall, or go for a run, or any other number of more useful activities when the Internet has ceased to entertain you.
But instead, like a gambler at a slot machine having another spin, you'll click refresh again, waiting to see if the magic 'entertain me' button has started working again. Even when the archives are a pretty darn similar type of button, you still stick with the one you're used to.
It looks pathological when you see old people at the pokies in Vegas at 3am doing it. But they're on the same hedonic treadmill as the rest of us - mine, for instance, just take place in private.
Part of the impetus for all this, dear reader, is that posting is going to be light for the next month or so, until late August. I have the distinct pleasure of roaming around Europe, in a kind of 'working holiday' type arrangement. Pessimists would forecast that the amount of 'work' in the 'working holiday' will be analogous to the function of 'massages' at a 3am massage parlour - it's the fig leaf that gets you in the door, but nobody takes it too seriously. Still, the only obstacle is my self-control, so we'll see how it goes. Pessimists might further note that describing a lack of self-control as the "only" thing stopping me working is like saying that the only obstacle to me winning the 100m freestyle at the olympics is that I can't swim fast enough. I fear that the pessimists may be right on both counts, but hope springs eternal.
But the moral of the story is that if you're a new reader, I encourage you to dig around the archives on the right in the mean time. At a minimum, I guarantee you that it's no worse than the crap that gets dished up here normally!