Construction companies fear finding historic artefacts much more, because the bomb squad works a lot faster than the archeologists. It's not that big a risk anyway, those bombs and grenades didn't go off as they were supposed to the first time, why would they go off now? People do get evecuated sometimes, but usually just as a precaution, with a warning well in advance and without them fearing for their homes very much.

And it's not everywhere, heavy fighting was usually limited to specific areas. There are parts of Belgium and Alsace where getting rid of shells is routine for farmers because of the enormous numbers of shells fired in WWI. And I found a handgrenade once as a kid when we were living near Arnhem (Market Garden, a bridge too far) and took it home. But it's not like the whole of Europe has been carpet bombed for the whole duration of two world wars.