Because other people matter and building a caring and just society means we’ll all get further in the one life we have. If you require the threat of eternal punishment to do the right thing and be a good person I’d question if you truly are.
Of course not everyone can or will do the right thing without being threatened with punishment... but I think it's possible to make the world a better place in general if we can at least tone down the amount of bad things happening, even if it requires "threatening" the people that need it to behave.
But if there is no afterlife, why does building caring and just society matter? If you (and everyone else, and the universe) is going to die, you might as well train your conscious and trample on the lives of others if it leads to success in life.
> But if there is no afterlife, why does building caring and just society matter?
Because some (most?) of us are empathetic and compassionate people. We care about others and hurt when they hurt. We want to live in a just society and want to leave behind a just society for others after us.
The fact that it requires „training your conscience“ to „trample on the lives of others“ suggests that such an outlook is not a default state of being.
Most of us are egoistic a-holes, and most of us like to build false picture of them in their minds so we can have nice time thinking that we are good persons.
