You should also take a step back and wonder why architects and historians aren't asking exactly the same question, or are satisfied repeating trite, obviously incorrect "explanations".

I would suggest the conclusion is that the forces responsible for the destruction of the society that created these structures all across the globe does not want us consciously aware of that destruction.

Further, the fact that the silence and obfuscation persists to this day is evidence that the same forces of destruction just mentioned are active to this day.