Friends :)

Their operating system. Their choice of components.
For quite a bit less than your average MacBook pro would cost, I built a gaming PC that completely outperforms it.
ASUS Z87-Plus with an Intel i5 quad core processor
Two ASUS GTX660 graphics cards
Western Digital Velociraptor 15k 500GB Hard Drive
250GB Samsung SSD
Antec 900 tower and matching power supply
Final cost of this masterpiece? $1,600
Skyrim on ultra with over 300 mods and many ENB's and graphics enhancements? No fucking problem.
Could a MacBook Pro handle that? Nope.

Solid aluminum case. That's what you're paying for pretty much. The only way Mac's are better than PC's is their case, but it's not $400-$1400 price increase upgrade
You're pretty much paying more for a name with lower functionality

You should have went with a single, more powerful graphics card than a 660 sli. But oh well.

The division of the workload between two graphics cards (and keep in mind that one 660 can handle most things perfectly, so more power isn't really even needed in the first place) garners you a more dramatic increase in overall power than a single, more powerful card. Plus the price puts two 660's lower than the more powerful cards I looked at

That would be true if adding a second card was equal to double the performance, but irl that is not the case. You can win up to 80% more power, and even then you are counting on the game or software that is using them to properly do it. Which hardly ever happens.
Going for a single card rather than dual o or more is always the best option, unless you are running 3+ monitors.

I got my MBP for $700

Yeah, 3 monitors is my minimum

FYI I put windows 8.1 on my MacBook and it was running new Vegas with 358 mods at 60 fps

So you buy a MAC to install Windows..

No I bought a Mac to have hardware that'll last 10 years, then I got windows 8 so I could game without using WINE