There are Windows containers and I don't think that most developers using Docker on Windows are even aware of them.

IMO it's kind of good to have Linux as a unified container platform. It's easy to run Linux application in Windows or macOS VM. It's hard form the licensing perspective to run Windows VM and it's very hard to run macOS VM.

I think it's just that Windows Containers are a lot unlike Linux Docker containers; they're not useless, but they're very different. I don't think Darwin containers have to be so different from Linux containers, not the least of which because Darwin is UNIX. It would be a lot more like if you ported Docker to run natively on BSD than Windows, since it basically is that.

An easy solution to the licensing issue is just having an open source userland that you can use, which Darwin sorta does. That said, since XNU syscalls are not stable, there is some awkwardness here. Either they would need to be made stable somehow (e.g. by providing something like personalities, where different processes get different syscall behavior) or the runtime would need to drop libc/base libraries in from the sky and containers would just need to assume they're available at a specific location. Either thing seems quite viable if Apple wanted to do it, IMO.

We are very much aware, they are commonly used on deployments used in Windows Server.

Typical examples, Sitecore, Sharepoint, Dynamics, Optimizelly, COM services, SQL Server.