Some of us aren't willing to spend $1500, just to use an os for compiling on, when spending $1500 to build a pc can build an absolute monster in any comparison.
But enough people are -- Apple is a hardware company first, software second.
At one point in the past they did license the Apple hardware designs to a 3rd party company to be able to produce them, and the company nearly went bankrupt over it because they couldn't compete with them.
Apple knows that allowing/enabling outside parties to create Macs will cost them dearly, so they are attempting to keep it all locked up.
It was easier to do when they weren't intel based, but they kind of had to make the jump because the non-intel chips got increasingly further behind the times and not getting on board would mean that they would sink anyway.
Anyway -- the biggest problem that I have with Apple is that they don't have any middle-of-the-road machines. You can choose between 'greatly underpowered' and 'ridiculously overpriced', but hardly anything half-way decent that won't break the bank.