The question posed was why do they Have to? Not why Should they.
Of course we need people driven to move society forward. But there is a stigma in modern society it seems around people who are perfectly happy being who they are and not "striving to be great". And that's ridiculous.
The previous poster asks all his potential dates, with what seems to be a fairly heavy helping of disdain for the "wrong answer", what their plan to change the world is.
Why does it matter? You go change the world. Let your spouse or your friends or your colleagues be content with not changing the world and just be happy living in it.
I'm not saying this is wrong or right, but perhaps some people feel everyone has a duty to do something meaningful to push society forward, otherwise they are essentially free-riding on everyone else's sacrifices.
And that something may very well be as broad a goal as "do your very best, everyday".
Sure, you can't force people to see the world through that same lens, but you can certainly pick partners based on traits which you admire, ambition / grit presumably being one of such traits.
Not speaking about (or judging!) anyone individually, but because if none of us did, we'd still be living in caves. Or, more likely, be extinct.
The reason all of us are having this conversation now is because many people before us had purpose and built something