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.
Otherwise I think laws wouldn't exist.
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.