Perhaps the simplest explanation is copyrights and patents. Microsoft and Apple are both known for their aggressiveness in suing those they think have stolen their intellectual property. They've even sued each other, multiple times.

Having said that, Apple and Windows stuff is a great deal more compatible now than it was 20 and 30 years ago. In the mid-1980s, a PC wouldn't even be able to read a Mac floppy, and vice-versa. PCs used IDE, Macs used SCSI. PCs used Intel/AMD CPUs, Macs used Motorola CPUs. Even the mouse and keyboard plugs were different. PCs and Macs couldn't even read each other's files. Most common file formats of the time were either Mac-only, or PC-only formats.

Today it's all a great deal more interchangeable. Both use SATA drives, USB mice and keyboards, DVI or HDMI monitors, the same Intel CPUs, the same nVidia or AMD GPUs, and most file formats now are pretty much universal, readable by Macs, PCs and Linux/Unix alike. The filesystems and OSs are still incompatible, something both companies are loathe to give up, so you still can't run a single executable program on both platforms (except via software or hardware emulators and the like).

But a lot of Mac hardware components today IS PC hardware. Especially aftermarket stuff.

And both companies want to keep their stuff proprietary and incompatible for business advantage.

There used to be a lot of DOS clones you could buy, back in MS-DOS days. But there's no Windows clone. Microsoft wants it that way to maintain market dominance and all those billions of bucks. Apple is the same. Both got rich. Compare with Linux, which is open source. Tons of linux variants, and the original writer of Linux, Linus Torvalds, still has to work a day job to pay bills, unlike Jobs or Gates who both became billionaires. So that's a disincentive towards mutual compatibility.