Is because the cost of an CPU, within the server, And the cost of Server within the whole DC, and then the cost of Running the DC. In the grand scheme of things CPU cost is pretty small. Not to mention we are getting another round of CPU upgrade with Intel Broadwell-E. And then some price competition from AMD's Zen.
And Intel Xeon isn't just about the CPU, it is the enterprise grade QA and testing, Network Controller, SATA controller etc. And the Xeon-D is a very Decent offering.
Then there is scale, even though Apple's Cloud is huge, it is still no where near Amazon AWS scale, not Google and Microsoft Azure either. And even if you add all these three Cloud Giant Combined, I will bet their annual buying CPU core doesn't even come close to Apple's annual iPhone sales.
So to finalize, there just isn't any financial incentive of switching.