My guess is that Apple buying ARM is a money-losing proposition. Actually licensing the ISA can't be terribly profitable for a company of Apple's size, and ARM being fabless means there's not much competitive advantage to owning (rather than licensing) the ISA. Simply owning ARM might invite antitrust concerns that Apple really doesn't want to be dealing with, and risks souring their business relationships. Actually trying to exploit the ownership, beyond collecting royalties, would guarantee both. As, perhaps, would any future efforts to put proprietary ARM extensions into their Cortex line.

So the reason Apple would buy Arm is to save itself money on the architecture license in the long run, right? But it could also save money on the architecture license if it switched to RISC-V. And you can be sure there's a tightly guarded spreadsheet somewhere inside Apple that models whether or not switching to RISC-V is a good idea.

Of course, Arm can figure all that out too, so somewhere inside Arm they've tried to replicate Apple's "should we switch to RISC-V" spreadsheet, and Arm sets the price of the architecture license so that the spreadsheet will always say "don't switch to RISC-V".

The other thing to consider: Arm's core design business is quite valuable; probably more valuable than their architecture business. But Apple does their own core design, and would not be interested in selling core IP to others, so they would get no value from that business. But if they were to buy Arm, they'd still have to pay for it.

Apple owned 43% of ARM in the early 1990s, when the company was investing heavily into Newton and associated mobile technologies during the Jobs interregnum.

Owning the instruction set and reference designs maybe isn’t terribly useful, when your primary competitors already have perpetual licenses to the technology and do their own core designs.

I think they’d struggle to do anything with it, given anti-trust concerns

Apple is one of the parties which founded ARM

This will make a huge headache for them and it's legal department since acquisition must be approved from government.

That requires that Softbank is willing to sell.