Answer:
So the Lord said, “I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air . . . ” (Genesis 6:7).
For the answer to this question, we’re going to get a little lesson in geology (the study of rocks, canyons, and mountains). If you visit the Grand Canyon, there is what we call “basement” rocks that are a mile down into the canyon. These rocks are amazing in that it looks as though someone took a really sharp knife and just sliced the earth right off . On top of that you see layers of rocks, sand, and mud that were laid down by water right over the “basement” rocks. This gives some understanding of how devastating the Flood really was. How it just wiped out the surface of the earth. The Flood was cataclysmic—it was dreadful, tragic, a disaster, devastating. It destroyed everything and everyone on earth just as it was intended to do. So there wouldn’t have been any buildings left from before the Flood.
Source: https://answersingenesis.org/kids/noahs-ark/after-the-flood-were-there-any-buildings-left/