One of the largest problems with moving to anything other than triangles as the rendering primitive is the decades of industry investment in pipelines for producing and hardware (on the player end) designed and optimized to push triangles. There would be huge issue with backwards compatibility for a GPU that defaulted to anything other than triangles, not to mention the graphics API’s designed for those triangle pushers.

Nurbs are not efficient for realtime rendering.

There's no reason to encode all that curve data if all you care about in the end is a finite set of pixels rendered on the screen.

You can achieve the same level of detail without having to recalculate the surfaces. It's a lot easier for the computer to morph a shape by applying some math function (that's essentially arithmetic) to all the polygon's vertexes.

That said, I've seen some 3d artists work in NURBs, then render out to Polygons.

GPUs have implemented tessellation shaders for a long time now, and they can be used to implement what you describe. The Xbox 360 already had them, but it was too slow at the time so not used much. If I'm not mistaken, later generations have used them extensively.