I don't know, ask Jonny Ive at Apple. If they had free reign to design their own SSD, they'd have much more say to design a even thinner iMac chassis. Instead of a bulky 3.5" industry standard drive taking up a chunk of room dictating the innards of the chassis, they could create all sorts of shapes to fit in any available space. The chassis could then be better optimised for heat exhaust airflow and therefore they could bump up the TDP of processors and or GPUs furthering performance. So while it may not be something wanted directly by consumers, it would allow for things consumers are more interested in, i.e. a more attractive outward design and better CPU/GPU performance.