The margins for those cheap PSUs have always been quite ugly because so many of the costs don't scale well, things like packaging, inventory, customer service, advertising, shipping, and so on. And with aftermarket PCs using a lot more power, you have fewer customers of those cheap PSUs you can eke out small gains with.
This isn't just PSUs, but all PC parts, really. While there's no doubt some old-fashioned greed involved, this is one of the causes of the entry-level GPU market disappearing; integrated GPUs have gotten good enough to bleed off much of the market for these cheap discrete GPUs with small margins, which causes the GPUs to be more trouble than they're worth for most companies. You'll still see these types of GPUs with prebuilts, but it *is* worth the effort when you know Dell or HP will buy a whole boatload of them from you.