Well, you do get a lot of power. In the end, its all about balance and about the definition of what is still "mobile". Sure, you can put 150W components in a portable chassis, but that will cost you a lot in terms of other design parameters. Apple has traditionally designed their high-end mobile computers at around 90-100W TDP max. I guess that is some sort of magical number that Apple is comfortable with in a mobile platform. So within this constraints you get fastest CPUs money can buy currently (there are very few other laptops shipping with high-end i7 Kaby Lakes, and I don't think that among those there is a laptop that can match the Apple's portability) as well as a "reasonable" 2TFLOPS of GPU performance.
As to desktop vs. mobile, doesn't it all boil down to size constraints? Desktops can afford to host power-hungry components, while a mobile platform doesn't. The difference in performance between mobile and desktop is as small as ever, but in terms of power delivery and heat dissipation desktop is simply superior. And yes, you can build a portable platform using desktop parts (Clevo does for example), but that is a very specialised market and not something Apple ever showed interest in.
P.S. Maybe it would make sense to distinguish between "portable" and "mobile" in this context. Portable as in "can be moved and set up on a new place with ease" and mobile as in "can be comfortably used on the go". The MacBook Pro is in the later category — its plenty fast, but not so fast that it would become a physical burden. The 15" MBP for example fits in a compact bag designed for 13" laptops while being significantly faster than the majority of other 15" laptops on the market and offering top-in-class battery life.