Germany and France were heavily bombed by the allies during the war. Some buildings were left destroyed as memorials of the war such as the Saint-Nicolas church in Hamburg or the Saint-Etienne-le-Vieux church in Caen (France).

In France, many cities were almost completely destroyed such as Brest, Saint-Nazaire, Lorient, Le Havre, Saint-Lô (called the capital of ruins because 90% of the city disappeared), Lisieux, Rouen, Caen... As well as in Germany : Dresden, Hamburg, Berlin... When you compare plans or pictures of these cities before and after the war, you can see that the structures of the cities changed a lot! The most important at the end of the war was to build appartments, houses. People needed to sleep somewhere and building again a monument was really not their priority.

BUT! There are of course examples. The Charlottenburg castle in Berlin was rebuilt and open in 1966. In Brest, France, the Naval Monument, a memorial for the US Navy, was destroyed by the Germans during the Occupation and rebuilt in 1958 to finally open in 1960.

Not WW2 but WW1. The cathedral of Reims was bombed in 1914 and got on fire. It has been rebuilt.