Comment by adrian_b

The obsession with triangles comes from Microsoft, who around 1995 chose to support only triangles in DirectX.

As a consequence of that, the GPU vendors designed then GPUs supporting only triangles in hardware, abandoning earlier attempts to also support other figures.

That has been inherited until today, so any program that wants to use not only the general-purpose computing features of a GPU, but also the dedicated graphics hardware from it, must convert any other kinds of graphics primitives into triangles, before rendering on the GPU.