Some rendering engines do smear time in various ways like Motion Blur. Ray tracing can pull it off more cheaply than rasterization but you still need to compute geometry multiple times in as single frame.
Depends on the specifics, but you can burn basically unlimited processing power shooting more rays and there’s just diminishing returns. We’re not there for games today but yesterday’s render farm is tomorrow’s GPU.
As to why it’s used, higher FPS creates artifacts on things like spinning tires. Persistence of vision means flashing something for 1/240th of a second can be surprisingly obvious. We perceive fast objects as actual blurs not just sequences of still images.
