Great question, but pretty much a completely different use case than the OP. The whole point of this project is to make it easy to run macOS on foreign host operating systems, presumably because the intended user does not own a Mac.
If I had to venture a guess as to why macOS doesn't support its own containers in any way, though, I'd say:
The core use case for containers is deploying apps to servers, and macOS isn't really used on the server. Apple themselves abandoned supporting macOS as a server operating system many years ago.
Sure, but Apple evidently doesn't view that use case as central to macOS' purpose, and the likely reason for that is that enterprises show little interest in running macOS as a general purpose server operating system. People are not running their web apps or game servers or storage networks or transcoders or scientific supercomputers or Kubernetes clusters or videogame lobbies on macOS servers.
This isn't new, and Apple has rolled with it; they gave up making server hardware more than 10 years ago. They finally completely EOL'd the software package they called 'macOS server' like a year or two ago.
Compare all the macOS changes from a 2 year period to the equivalent changelogs to the Linux kernel. It's very clear that macOS is neither widely used as a server operating system nor developed as one.
If macOS were widely deployed as a general-purpose server operating system, Apple would have implemented some kind of container support years ago.
This isn't the only server-centric area in which macOS lags compared to other operating systems. APFS' featureset is anemic for such a recent filesystem with a copy-on-write design.
The core use case of containers is central neither to existing macOS usage nor to Apple's vision for macOS. If there's a question here, it has to be why that is so, not the fact that it is.
If I had to venture a guess as to why macOS doesn't support its own containers in any way, though, I'd say:
The core use case for containers is deploying apps to servers, and macOS isn't really used on the server. Apple themselves abandoned supporting macOS as a server operating system many years ago.