Somehow ARB, and now Khronos keep forgetting that having something like MetalKit or DirectXTK as part of the specification really matters to onboard newbies.

OpenInventor could have been it, but SGI had other plans for it, and no one at either ARB or Khronos actually cared about it.

It isn't just newbies, i write OpenGL code for almost 2 decades, i see no reason to do use the harder parts of the API when i can simply use the easy parts :-P.

Though i disagree with OpenInventor, it is too complex and takes too much upon itself, which few wanted - see Direct3D Retained Mode which despite being a much simpler API (both compared to D3DIM and OpenInventor) didn't see much use and in an uncharacteristic move by Microsoft (especially at the time) it was removed from Direct3D.

A better solution would have been something like GLUT, but with a few more utilities thrown in (like vector math stuff - OpenGL implementations already have the code anyway, why not expose it?). And GLUT was more popular than OpenInventor ever hoped to be despite not offering more than a few basic things.

I think that widespread adoption of middleware engines has proven that retained mode was the way to go.

The ones that don't use one, end up reimplementing their own scene graph anyway.

As for the rest, full with you.

That middleware is a different beast though, they are different engines providing different solutions for different problems - you can't have any single one of them be the solution to all problem and most of them are very complicated to be defined as a standard that is supposed to live for decades.

OpenGL, Vulkan and Direct3D are at a level where they enable you to write your own engine, but not at the level where they provide your the engine themselves. Microsoft tried it with Direct3D RM and it didn't work and Sun also tried it with Java3D as the official way to do 3D in Java but also didn't catch on (it caught on more than D3DRM but that is mainly because there was no other official way - however its popularity paled in comparison to OpenGL bindings that appeared soon after and nowadays it has been reimplemented and lives on top of these bindings).