Engineers don't collect royalties on bridges in the first place, unless they have some insane contract. They don't own the bridge, they designed it under contract for people with lots of money (who do own it). The children of the people who own the bridge can, in fact, keep collecting money. The Rockefeller family is still tremendously wealthy.
Copyrights are legally property, the same as sports cars or collectible stamps or gold bricks or bridges. If the parent dies, he can leave the copyright to his children in his will, same as any other property. This ability to sell the rights to the song is responsible for basically every single piece of culture you enjoy. The Beatles sold the copyright to their music: it wouldn't have been practical for them to sell tapes of their music after their shows. The reason they were able to sell music in the first place was because of the copyright system.