ARM hasn't. Apple has. Apple's implementation of the ISA since A6 is unrelated to ARM's.
Also Intel and AMD have not delivered higher performance via more cores; they delivered lower price for (say) 64 cores worth of performance, by putting them all on the same chunk of silicon (edit: or at least in the same package). (There are some slight improvements in inter-processor interrupt and cache-forwarding latency, but if that's a performance bottleneck, the problem is bad parallelization at the code level.)
Have you looked at the encoding of Thumb-2 (T32) and particularly A64 (their newly designed instruction set for 64 bit)? Their instruction encoding is in my opinion much more convoluted than x86.
Arguably AMD did the exact opposite - lower prices via splitting a processor into multiple pieces of silicon. (Chip price scale exponentially with area at the high end)
Sure, but that's them having a competent
(well, less incompetent) ISA and microarchitecture
Intel and AMD might be able to deliver somewhat higher performance by throwing a whole bunch of cores at the problem, but they do so at a much higher cost in power requirements. And it would be easy enough to design ARM machines with an equal number of cores (or even way more), and still have much lower power requirements.
Intel stagnated, and has high power requirements. ARM has caught up, and has much lower power requirements.
The reasoning seems pretty simple to me.