Not all of them know that they won't matter after their death. Some people talk about procreating to create a legacy and these people genuinely think that they'll be remembered 100 years down the line. Though, I'll admit that perhaps some normies probably acknowledge that they don't matter in the long run but still participate in things like self improvement because they exist right now and they want to make their life as enjoyable as possible. Sure, they may not matter after their death but they still want to have the best life possible whilst they are alive