Answer is simple, its property of material.
I am fan of ancient alien series, they show all old monuments build on earth.
And they use to decode the scripts writen on those monuments, do you know why?
The history people date back everything by doing carbon dating so why they decode scripts writen on them?
Because stone is a material which cant be carbon dated.
It does not decay over time - it may have some decay due to weathering but we can not know about its original date of making.
We use steel, concrete etc this material itself have expiry and with time it becomes soil (i saw in one series that in period of 5000 years steel get decomposed in soil)
So even if human get wipe out from this earth, all our creation will merge back into soil and even if someone come from another planet, they cant find any advancement ever happen here, because nature is more powerful than anything.
Hope you would have got answer for your question.
100 year old home made with stone is stronger than 50 year old concrete structure.
Any structure which is designed by todays advance structural designer does not state life of more than 100 year. If it is so then how one can expect it to last long.
Max structure design with life span of 60 years as in this period designer and intended user may get vanish or become only senior citizens who need rest.