That assumes someone would buy them as an ongoing business. Apple could have bought them specifically in order bring the ARM designers in to Apple and force their rivals to switch - an acquihire on a massive scale. Using an ARM designed chip is an advantage, and taking that advantage away from your competition could be a very good thing for your business.
Apple completely design their own chips. The real mystery is why Apple stuck with the ARM architecture when they switched to 64 bit (since aarch64 is a brand new design). My guess is because they still had to license the 32 bit architecture for compatibility, and because the amount that Apple pays to ARM each year for the license is peanuts.
For the same reason they haven't switched to ARM for macOS yet - compatibility. If Steve Jobs was still alive, he would've probably switched macOS to ARM by now, despite all the hassle. He's done it before. I'm not really seeing Tim Cook pull a move like that anytime soon.
I'm glad that didn't happen though.