alt Hacker News> so when rebuilding it was probably just easier to just leave them there. That's just a guess.
My take (equally guessed): Among the literal rubble spread all over the cities, people constructing everything back up with whatever means possible couldn’t even recognize a mostly buried object as a bomb (instead of just more scrap from a collapsed building).
So less of an active decision to not deal with it, rather one out of necessity and/or not knowing about them, perhaps?
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...