Because we do. We mourn and go on, and we expect to obediently die when our time is up. Your whole comment is the reflection of that custom.
If it weren't for it, then any reasonable human would be doing everything in their power to get rid of death entirely. As a civilization we'd be probably already done, but we aren't, because almost everyone is used to death as a "natural order of things" and/or delegates ending it to a deity.
> Human beings have no right to even want to live indefinitely until they have figured out how to live the limited time we got here without fighting and killing each other, how to live and expand without reckless destruction of nature/environment, how to expand life to other worlds, planets etc etc.
Human beings have all the rights they give themselves, there's no external entity to tell us what we're allowed or not allowed to do. Maybe you want to die. Go ahead then, nobody will stop you. But I, and many others, don't. Living is better than being dead. And it doesn't mean being in conflict with limits of our planet, maybe if people lived longer than ~80 years they'd actually start thinking about the long run, since they'd have a stake in it. Part of the reason we're exploiting the planet so much is that because we're not going to be around to see the consequences.