Well, you are assuming that there are boatloads of people who want to buy Macs. Have you ever thought that most people might be buying PCs because they love them? To entice those people Apple would have to lower the price below PCs and we all know this is not going to happen. Besides, having high prices is Apple's marketing strategy. They cater to a category of people who like to brag about owning something others can't afford (I am not saying that all Mac users belong to this category).
Let's say Apple allowed its retailers to offer the Mac mini for, oh, $400, the entry-model iMac for, say, $700-800 and the MacBook for about the same. I honestly think you would see sales go up very, very high.
The people who are going to buy the $300-400 PCs, won't likely ever consider any of the iMacs or MBs/MBPs, but at $400 they'd probably consider a Mac mini.
There's also plenty of people who'd like to probably give an Apple system a try, but normally settle for an $800 PC because that's what they can best afford. All of a sudden, on Black Friday they see the iMac being offered for such: I think they'd jump on it.
People here (usually the elitists/etc) tend to scoff at such ideas, saying "well, anyone who's buying a $800 PC should be able to easily afford $300 extra for an iMac), but the reality is, there are plenty of people for whom that extra $300 represents half a pay check (or even less).
Overall, I don't think it would have hurt Apple to have joined in and at least offer a few nice discounts on, say, the entry models. But, as others have mentioned, Apple offering just about nothing is par for the course. At least as long as I've been a fan these past 12 years, lol.