My own guess is that a fair number of them just buried themselves. While I don't have all the relevant facts at hand, per the US Naval Institute, a 550 lb bomb dropped form 20,000 feet would hit the ground at 880 feet per second [1]. If my rusty mind is correct, that would be around 9 megajoules, while feels like it ought to be enough to bury a bomb pretty well in any sort of yielding soil.

1. https://www.usni.org/magazines/proceedings/1941/december/how...