Well, maybe there are only so many monoliths to go around :)
Seriously, this the second Fermi-paradox article I have read this week, and it’s very interesting that both of them consider an incredible variety of highly speculative solutions, while deliberately ignoring the most obvious possible explanation.
“O foolish people, and without understanding; which have eyes, and see not...”
Kubrick originally wanted to do Childhood’s End but somebody else already had the film rights. Perhaps intelligent species evolve into something non-physical.