Bombs that failed to detonate tended to bury themselves into the ground. In the chaos of a bombing raid where thousands of bombs might be dropped, it’s not practical to track which bombs landed where and which ones didn’t detonate.
So they just didn’t necessarily know at the time where all these bombs were, the ones they did know about would have been dealt with eventually, but the ones they didn’t know about? Well, large area underground clearances at that scale (like, your whole city scale) aren’t practical.
Fun fact, the US military EOD school is located on an old Air Force bombing range. They still occasionally find bombs there, sometimes it even gets students a day off.
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