Tooth decay is the only reason to brush your teeth.
Imagine a kid who doesn't want to brush. If they don't brush their teeth those teeth are still there in the morning, so why would they 'believe' in tooth decay?
If my teeth never decayed, why would I brush them?
That's the kid, a 12 year old who's teeth are fine, even when he doesn't brush. He can test that by skipping brushing every day. The consequences won't catch up until their early twenties.
The only immediate reason to get into the habit of tooth brushing is because they're told to do so, and if the lesson sticks then they don't have to experience the consequence of tooth decay as they grow older. The decay isn't a punishment dealt out as judgement for not following an arbitrary rule.
shouldn’t just start being a greedy asshole regardless.
Based on what? Greedy assholes have a great time! You certainly can't simply assume that everyone has a moral core that aligns with your own and have that match reality.
Naturally good-hearted people don't need compassion explained to them, if only everyone was like that, right? The suggestion 'learn to get along with people while you're alive or you never will' is certainly true whether there's an afterlife or not, but if there is then it's even more compassionate to try to give greedy assholes a framework and a reason to try something else.