the way Waymo does it with the spinning sensor

There's a reason they do that, he actually covers that in the video. Lidar spins a single line many, many many times a second. Processing the differences in that line scan to scene makes the point cloud generation many times easier allowing the scan to be exponentially more dense.

The iPhone uses a diffraction grating to shoot static dots at your face and looks for the subtle movements of your face and phone to generate a 3D scan.

The diffraction method is tiny and good for static identification but bad for high-speed outdoors.

The spinny towers give it a better field of view. you could probably put shorter towers on each corner, or even build them into the body panels, but it's a delicate, expensive instrument and it's not what's currently holding back self driving anyway :)