Mostly, I'm surprised that Google isn't the one leading the charge here. Although it's kind of cool from an ecosystem perspective to see somebody else moving things forward.
Is Google using ARM for their custom TPU systems? Perhaps they have some sort of non-compete agreement with ARM
TPUs are custom ASICs.
They might be worried that it would fragment their app store, as plenty of apps use native ARM code
Android used to support MIPS and x86_64 - in fact I think it still supports the latter. Of course, that might just mean that Google is painfully aware of how annoying that is to handle:)
They do, as well as x86 (32-bit); they are targets for emulator system images.
Not only that, there were some x86-based (but not PC-BIOS) devices running Android out of the box.