Have you seen the insides of one of these things? If you did, you'd be surprised at how much hardware they can fit as it is.
A MacBook Pro 13 Retina comes with: Not one, but two ThunderBolt ports, an HDMI port, two USB 3.0 ports, a bluetooth chip, a WiFi card, an audio out and an SD card reader. The logic board, which is literally the heart and soul of your MacBook is this small:
For size reference, on the left is one of the USB ports which is around 15.7 millimeters. This makes the logic board approximately 80x160 millimeters squared, or rather, 3.15"x6.5" or so. Do you have any idea how small that is? That's about half the length of your keyboard and around half the width as well. Remember that discrete graphics cards also have discrete memory and that quad core processors, while the same slot, require more power delivery.
To be perfectly honest, it's amazing that Apple was able to engineer this much power out of such a small machine to begin with. Even building it to these specifications in the dimensions of what's as small as a Windows ULV notebook but with the power of a full powered 13~14" laptop is quite an engineering accomplishment.
If you really want more GPU power, you can easily throw an external GPU at it. If it's a 2013 Retina model (which has a UEFI capable bios), then external GPU's are plug and play. And I don't know about you, but I'd much rather have a GTX 770 desktop variant powering my Titanfall/Skyrim fun than a mobile series 750M or 850M.
