Because I associate an OS with more then just an environment. It often has several running apps for instance, often a GUI or shell (which many containers don't have), are concerned about some form of hardware (virtual or physical), and just... Do more.

Containers by contrast are just a view into your filesystem, and some isolation from the rest of the environment through concepts like cgroups. All the integrations with the container host are a lot simpler (and accurate) to think of as just simply removing layers of isolation, rather then thinking of it like its own VM or OS. Capabilities just fit the model a lot better.

I agree the line is iffy since many OS's leave out a few things of the above, like RTOS's for MCUs, but I just don't think it's worth thinking of a container like its own OS considering how different it is from a "normal" Linux based OS or VM.