> Shouldn't it be easy for the radar to detect if there are stopped obstacles ahead?

No. It's very difficult. Radar reflects off of everything. The only way to get a useful signal is to look for things that are moving at a different speed than you, like something moving relative to the road. So the radar software filters out all signals that have the same speed at the road ahead.

This means the radar won't see anything stopped on the road, because it's filtering out a huge number of returns from the road. This gets even worse because Radar reflects very differently based off the material. It goes right through plastic, water (humans) absorb it.

Even if you could try to read through the noise of the road, Radar doesn't have enough resolution to tell the difference between the road going uphill and a stopped firetruck.