It just takes one weirdo civilization that decides to put up Ozymandias-style monuments to themselves everywhere- they don't even need to do it themselves. They

It just takes one weirdo civilization that decides to put up Ozymandias-style monuments to themselves everywhere- they don't even need to do it themselves. They can fire off a few Von Neumann probes and let them do the work to carve Mount Rushmores into every rocky body they can find. It's not much weirder than things humans get up to, and if spacefaring civilizations that can build such probes are common at all (even if short-lived), you might expect at least one of them to have initiated such a project and been successful enough to have built a giant "we wuz here" radioactive pyramid on the moon. Evidently, none of them did- and depending on how many there were, this is either unsurprising or deeply unlikely.