I've read that quality is largely limited by optics, and optics are limited by space in the MacBook line (just a few mm). Not sure why Apple wouldn't improve it on the iMac (a few cm depth to work with!).
Reducing the size of electronics has always been a goal for consumer hardware companies. Look no further than the memory stick which now supports tens and hundreds of gigabytes on a piece of metal smaller than your finger nail.
It's quite nice having devices that have such small profiles. It's the result of incredible engineering to fit more power in a smaller space for really very few tradeoffs.