Deskputer: why don't we all have these?

Now this is a desktop computer. It seems like a pretty obvious idea, and nothing occurs to me off the top of my head that would make it impractical. A custom motherboard with rotated PCI slots, a few extender cables, and you’ve got a really great setup. There’s plenty of room for vents and heatsinks, a natural place for drives, ports, and so on — really, why aren’t these things all over the place?

It’s not under the “concept” category there (although it looks like a CG render), but good luck getting one in real life.