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i'd say there are two main reasons: things get covered by dirt, and things degrade way faster than you might think.

After a couple hundred years of abandonment all that remains of a structure will be rocks, those rocks will tend to scatter and make the structure hard to discern, and after thousands of years any trace you might have been able to discern with a trained eye is simply underground.
Like it's kind of insane, in many places you can be standing ontop of many many meters of archeological sites, just waiting to be uncovered.

Here's an example of the remains of a building that you would probably just walk past without thinking much about it, maybe just going "huh, weird".