Apple moved all of their devices over to their own ARM chips, and now it looks like Microsoft is heading the same way
Apple exerts complete control over the hardware and software in Macs, so it's relatively easy for them to switch to another hardware architecture. Meanwhile, Windows runs on hundreds, maybe thousands, of different hardware profiles and it would be extremely difficult to support that many different systems if they all switched over to ARM. This is due to the fact, as other posters have pointed out, that ARM is missing support in BIOS/EFI/UEFI to provide information about the hardware present on the platform. That is fine when the hardware manufacturer creates their own chips and has a semi open source OS such as Android to integrate with their hardware, but it's a lot tougher on a closed platform such as Windows. I don't doubt MS is working on its own ARM-based processor to release in an MS-branded device that attempts to compete with Macs, but I highly doubt MS will be releasing a general-purpose version of Windows that runs on all ARM-based laptops and desktops and has an emulation layer to run x86_64 apps with performance similar to what Apple pulls off with Rosetta 2.
We want to create puppets that pull their own strings. - Ann Marion