WWII Leftovers

Today, as we were driving to the grocer store we heard on the radio that a rocket launcher from WWII was found near the Autobahn. Aparentl they detinated the rocket and no one was hurt. This was the second time since I've been here (about a month) that I've heard of WWII weaponry being found in Germany. The first was a bomb that was aparently burried under the Autobahn. It was struck during construction and killed at least one person, injured others and caused a traffic delay. Aparently this happens from time to time in Germany. What I don't understand is how come, almost 70 years later, these bombs were not removed? Things were just built over them as they lay dormant in the ground with the possibility of explosion looming. Aparently they are under cities, towns, etc and it is too much of a hassle to remove them/it is not know where all of them are. This is the kind of stuff you don't hear about in America!