You said "why must Linux have" a feature that can be useful to some and not useful to others. Taking that to its strongest conclusion[1], you're saying Linux shouldn't have something if it's not useful to "everyone" and asking for counter arguments; this is not unlike the "Do one thing and do it well." Unix ethos.
Clearly, as demonstrated by history, most people prefer that their computers can and will do the many things they need or want with minimal finagling. That is what having DirectX inside Windows means, and why Linux which makes that a finagling option at best and flat out refuses as heresy at worst flounders.
> ... you're saying Linux shouldn't have something...
I said no such thing. You're taking a question and converting it into a statement in your own head.
Why must any operating system be designed with a 3D rendering engine compiled into it? It's just a question. I'm trying to learn. I've never once said it should or should not have the thing, I'm asking why would it need it? Why should it have an equivalent to Windows' implementation of such a thing? What do I gain? Is that always a good design choice? Is that true of Windows Server, and if so, why do I need 3D rendering baked into my Windows Server? What about Windows Server Core... does the NT kernel have it baked in there?
>It _can be_ useful. It can also _not_ be useful to others. It sounds like it's not a choice in this case, but a forced feature, and that's fine for some and not for others.
>So again, why _must_ Linux have an equivalent?
That is very different from simply asking why Linux should have a "Direct3D" built in like Windows does Direct3D.
>What do I gain?
To answer this again and more in-depth this time: A central, powerful subsystem that can be assumed to exist. We can assume Direct3D is and always will be available in Windows.
One of Linux's biggest problems is you can't safely assume anything will exist, in particular cases not even the kernel. This is the reason containers were invented, because you need to bring your own entire operating environment on account of being impossible to assume anything. The cost for this workaround is performance and complexity, the latter of which most users abhor.
>Is that always a good design choice?
Yes, it enables users thereof.
> Is that true of Windows Server, and if so, why do I need 3D rendering baked into my Windows Server? What about Windows Server Core... does the NT kernel have it baked in there?
If the server is a media server, say, having DirectX means the server can do encoding and decoding itself and that's something many people want.
Windows itself also needs Direct3D for rendering the desktop, which Server also obviously has.
Clearly, as demonstrated by history, most people prefer that their computers can and will do the many things they need or want with minimal finagling. That is what having DirectX inside Windows means, and why Linux which makes that a finagling option at best and flat out refuses as heresy at worst flounders.
[1]: https://news.ycombinator.com/newsguidelines.html