We already have way more money going into entertainment than into space exploration, it's not that hard to imagine that all sentient species eventually learn how to hack themselves into permanent happiness, and then conclude they have no reason to risk death on the final frontier.
This ofcourse requires some projection of our ideas of happiness onto unknown alien beings, but then why not also project boredom? If anything, we have seen that nothing in nature really stays the same. It is ever changing and evolving.
If those probes are Von Neumann machines they can spread throughout the entire galaxy in as little as a few million years. That's the crux of the Fermi Paradox - on galactic timescales any sufficiently advanced civilization should be able to expand its presence to the entire galaxy pretty easily and quickly.