I learned all of this the hard way. MSI is a perfect example of what the previous 2 posters are talking about. They offer some pretty impressive specc'ed PC notebooks for the price on paper (just look at the graphics cards in their $1500 notebooks), but the fact of the matter is (at least in my case) it's truly a matter of you get what you pay for.
I currently have a 2 yo MSI notebook (GX660 originally bought for $1,299 USD, you can look up the specs) that is starting to break down. The Wifi card is a poor quality single band 2.4 GHz card that, while it meets the specs you are looking for on paper, it has weak signal strength, slow download speeds, and is normally kicked off of my network when more than 4 devices connect to it.
It features 2 USB 3.0 ports but if you benchmark them their throughput is slower than comparable notebooks. What's worse is that one of them has become wonky and will sometimes not recognize devices attached to it.
The optical drive also has a mind of it's own. Sometimes it will read DVDs, but often I have to eject and re-insert the disc just to get it to read. After 3-5 tries these discs will eventually load up.
Bear in mind this notebook has never been abused. It has spent it's entire life sitting on a desk, with an occasional road trip and handled with kid gloves. The bottom line is that -- while on paper the specs look good, the actual product's quality is certainly not up to my expectations for a 2 year old notebook. And I haven't even mentioned the subpar LCD display and the cheap plastic housing on this notebook.
This all said, having bought its replacement, a rMBP, the quality is like night and day. The WiFi card in the MBP is dual band, and amazing. whereas I was pulling maybe 3 Mbps on my MSI, the Macbook is averaging 5.6 Mbps. The display is light years ahead of the MSI. And the construction and build quality is heads and shoulders superior to any PC notebook I've owned previously.
So to answer your question "why is the 15 inch MacBook Pro so expensive", as mentioned above all you really have to do is physically compare the build quality between an Apple and most any other PC manufacturer out there. Compare the quality of the screens, the fit and finish. And definitely play around with OSX at an Apple store. It's no contest. The MBP is definitely worth it.