The 680mx is quite an expensive GPU, I'm not sure how expensive but probably about the same price as a desktop 680.
A lot of people don't see the value in design and see the computer from a more functional point of view. In that regard, making it thin and pretty could be a mistake because it's form factor means reduced performance. Because of the all-in-one form factor, the 680mx is used instead of the desktop 680, even though they're about the same price. In terms of function, the form factor doens't provide functional benefits. In fact, the form factor also means sacrificing a DVD drive. If performance and function is that important to you then you can something much better with a desktop PC.
But many mac users use their computers for things other than gaming, or they value design more. The 680mx might not be as good as a desktop 680 but it'll run games fine for the majority of mac users. Some people buy macs and are happy with it because of the design and some people buy macs and regret it afterwards because they bought it for the design and they realize that it doesn't perform as well as a similarly priced desktop PC.
Do your research, buy a PC if it suits you. PC's are fine, I don't like how mac users hate on PC's or get all smug about their macs. But macs are really nice computers too. It depends on what matter to you.