Apple controls both the hardware and software. So, the software is built around very specific hardware releases. There are no drivers for unsupported hardware, and there is no troubleshooting for unsupported hardware even if existing drivers work.
Furthermore, Apple has a short cycle for end-of-life-ing old hardware with new software and OS releases. For example, even though my G4 iMac has a GPU that is supported by OS X, it is only officially supported by older versions of OS X, up to 10.4. I can hack the installer to install 10.5 Leopard on my G4 iMac, and for the most part it runs perfectly, but because of the old GPU I sometimes get weird visual artifacts after sleeping the computer. Obviously Apple won't do anything about it because it's simply not supported.
Now, extrapolate that to Hackintoshes. If Apple won't even support some of their own older hardware with current OSes, don't expect completely foreign hardware to work. You usually have to have very specific hardware that either is identical to what is in the Mac, or is very similar. This often means specific motherboards and chipsets that just happen to work with the OS.
So, why deal with all these disadvantages? Well, there are advantages too. The biggest advantage is that you spend a lot less time fighting with driver issues, etc. I find I spend more time troubleshooting my Win 7 installs than I do with my Macs, although both have their issues.