You save others bc it saves yourself from the grief and guilt and horror of watching them suffer and die. Think of it as saving your own emotional, mental, and spiritual health.
Do you actually know everyone? Don’t be ridiculous, of course you don’t/. You’re naive and clueless. There are many people who don’t try to save anyone but themselves.
Because not everyone is a nihilist.
because they care
Because they want them around for longer? And it’s nicer to die after living a long fulfilling life rather than die young with unfinished business.