Because the expense is not really worth it - even GPU rendering (while around 3/4 x faster than CPU rendering) is memory constrained compared to CPU rendering, and as soon as you try and go out-of-core on the GPU, you're back at CPU speeds, so there's usually no point doing GPU rendering for entire scenes (which can take > 48 GB of RAM for all geometry, accel structures, textures, etc) given the often large memory requirements.
High end VFX/CG usually tessellates geometry down to micropolygon, so you roughly have 1 quad (or two triangles) per pixel in terms of geometry density, so you can often have > 150,000,000 polys in a scene, along with per vertex primvars to control shading, and many textures (which can be paged fairly well with shade on hit).
Using ray tracing pretty much means having all that in memory at once (paging sucks in general of geo and accel structures, it's been tried in the past) so that intersection / traversal is fast.
Doing lookdev on individual assets (i.e. turntables) is one place where GPU rendering can be used as the memory requirements are much smaller, but only if the look you get is identical to the one you get using CPU rendering, which isn't always the case (some of the algorithms are hard to get working correctly on GPUs, i.e. volumetrics).
Renderman (the renderer Pixar use, and create in Seattle) isn't really GPU ready yet (they're attempting to release XPU this year I think).