its probably just what they found and used..i mean if
we went to the Moon and had to use what was there we
would have boulders with etchings on

Maybe they came here with high technology and knowledge
which they took with them when they left.
If
they had most of what they needed when they got here
then maybe there was no need to set up multiple harmful
industries that we have today, like plastic
manufacturing. Seems many people are thinking of ways
to do things a lot simpler these days, in more Earth
friendly and natural ways and live more like our
ancestors did. Also, there was a period in time where a
lot of old artifacts made of metals were melted down
and destroyed, as I believe is still going on today.

because rocks last longer...exterminate the human race
now completely and although with all the high
technology we currently posses after 200 thousand years
there will be absolutely no evidence that a human race
even existed. But the stone structures will still be
there.

Why should the aliens who evidently are capable of
building spacecraft, build monuments of stone and not
use metal and alien technology ?