Non-directional doppler radars are cheaper than the kind of directed radars that would be needed to map the distance to all static objects.
Also a dopler radar can scan the whole sector with every emission, and simply measure the distance to the closest moving objects (and their speed) with each scan.
A radar that creates a 2d image of the sector usually needs to send a targeted beam in one direction at a time. Such a radar is not only much more expensive, it also adds latency.
Visible light is better suited for 2D imaging, anyway.
Also a dopler radar can scan the whole sector with every emission, and simply measure the distance to the closest moving objects (and their speed) with each scan.
A radar that creates a 2d image of the sector usually needs to send a targeted beam in one direction at a time. Such a radar is not only much more expensive, it also adds latency.
Visible light is better suited for 2D imaging, anyway.