When the vehicle is stationary, the radar data has to be heavily filtered because radar has poor angular resolution and there are a lot of other stationary objects, many that you don't want to stop for at similar angles, like overhead road signs. That means you're relying purely on vision, which alone has a lot of false positives and negatives, so detection thresholds need to be set much lower.

Doesn't mean it won't work. Just means it's less likely to work, especially in the dark and the rain, where radar would normally be a big part of the signal.

Citation needed. Subaru claims their eye sight system will detect and attempt to brake for a stationary vehicle. Whether or not they’ll managed to halt the vehicle is obviously speed dependent, but obviously slowing down before the crash is vastly superior to maintaining speed.