Why are there still WWII bombs that explode in Poland today...?

Bombs dropped over Poland, and many other countries were far too many, and far too widely dispersed to be able to safely detonate all of them after WWII. Some bombs did not detonate upon impact, and, while the Polish Government/Military is trying to find and safely detonate them, the bombs are being accidentally found by and killing civilians faster than they're being detonated by the Military.