Detecting static objects in view using RADAR
It is because of ground clutter. Car manufacturers use Doppler radar. Doppler radar utilizes the Doppler effect to help the radar to distinguish objects that are close together. 


You can see how the moving object has different frequency and can be distinguished from the terrain even though the range return is identical. The problem with stationary objects is that they have the same Doppler shift as the earth, trees, signs etc. The radar cannot distinguish between a dead animal and the road.