Primary rays cache, secondary rays trash. That's why. If anyone comes up with a powerful enough system to do truly useful and fast raytracing in realtime, they will show it.
http://www.disneyanimation.com/technology/innovations/hyperi...
most animation or vfx rendering takes hours per frame.
The graphs in Figure 10 imply that the interior scene can be done in 10 minutes, but Figure 4 says the scene takes 68 minutes to render.
Almost everyone else is still doing pretty much shade-on-hit one-at-a-time and getting pretty good performance out of it.
PRMan RIS is batching shading points up, but after the first bounce the batches get much smaller, so the win you get from it is much less.
Disney do a lot of work sorting the batches, and that takes time.
Also their render times for full frames are generally 12 hours wall clock or more (like most of the rest of the VFX industry). And Disney are heavily de-noising their images afterwards anyway.