They have a lot more control over the mobile processors. The laptop processors have to maintain compatibility with all the software that people run on their laptops; effectively, that means they're stuck with other people's processors.
That's not really true. x86 emulation is a solved problem at this point, and Apple has shown willingness to use emulation to bridge an ISA transition in the past.
I think the real reason is some combination of (a) ARM isn't competitive (or only recently became competitive) at the high power/high performance point that Intel CPUs excel at; (b) the Mac line generates so much less revenue and operates at so much smaller of a scale than iOS devices that it isn't worth developing CPUs in-house for them, not to mention the fixed costs associated with undergoing a transition.
I guess Apple could 'survive' releasing a new generation of laptops which run slower than the previous ones to facilitate a user-invisible component sourcing decision, but it seems like a bad move.
It'd be temporary and only for third-party apps, not Apple-supplied ones. But yes, I do agree that it's not worth it for them--this is part of what I mean by high fixed costs of switching.
