What's With All the Repeated Commercials?

My wife and I tend to watch current TV shows on our computer, using the "full episodes" feature available on the networks' sites. No cable, no TiVo, all free, so long as we're watching the recent episodes (and older episodes are available cheaply for download, or via NetFlix if they're from a previous season). Some people don't like watching TV on their computers, but it works just fine for us.

Here's one puzzle, though: Though there aren't as many commercials as there are on live TV, the commercials tend to be all the same. We see a commercial at the start, then we see the same commercial ten minutes, then the same commercial ten minutes after that. It's not a huge annoyance, since they're just 15 to 30 seconds long. But I don't get it -- why would the advertisers want to bore us, even alienate us, by throwing the same commercial at us again and again? Even if we're paying attention the first time, we won't be the rest of the times. Why not give us a random mix of commercials, so that each one will be at least a little fresh?

I realize that people tend to ignore commercials, and that it may take several viewings for the viewer to absorb what's being said. But I'd think that it would be less annoying, and thus more effective, to have the several viewing be spread over several different episodes, rather than trying to get the viewer to like the product by giving him exactly the same pitch several times within an hour. I suppose I must be wrong, given that lots of smart people pay lots of money to play commercials this way. Still, if anyone has a more detailed explanation of the plan, I'd love to hear it.