Apple doesn't want to ship a computer with a crappy integrated GPU, because they want to ensure that users have OpenCL-capable GPUS. Intel is not offering i3s or i5s with good integrated GPUs, and Intel won't let NVidia make chipsets anymore, so in order to offer an i3/i5/i7 system with a good GPU, they need to use 3 big, power hungry chips (CPU+chipset+GPU). During the Core 2 era, Apple took advantage of the great combo of a Core 2 CPU + NVidia chipset with good integrated graphics to offer reasonable performance in smaller packages. They no longer have enough room for a 3 chip solution, so until Intel either improves their GPUs or starts playing nice with NVidia again, Apple faces the choice between using older CPUs or regressing to older enclosures and higher power consumption.