Apple makes a damned good mobile chip, but it's still just a mobile chip and it plays by mobile rules. Do you think apple has the expertise to switch to desktop class chip design?
A lot of the appeal of the Intel based Macs is the ability to run Windows at native speeds. That wouldn't be possible with ARM.
Finally, Apple only sells 16-20 million Macs a year. I can't see it being worth the investment to switch to ARM and creating all of the auxiliary supporting hardware it gets to use by using standard PC hardware.