Apple has made three processor transitions 68K -> PPC -> Intel. During each transition, the processor they were moving to was much faster than the previous processor. That's not the case for ARM vs Intel.

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.

PA Semi was developing workstation/server class POWER processors before they were purchased by Apple.