What value do you get out of believing in Earth Overshoot Day? Does it provide real, actionable utility? Does such a belief instill good feelings?
If I had to guess: it tickles the reward pathways in your brain, connected to your survival instinct to be aware of possible threats, whether real or (as I would argue is the case here) imagined. I would doubt there's anything you plan to personally do about Earth Overshoot Day, and I doubt thinking about it instills positive feelings. It's doom porn.