People with families can certainly be successful entrepreneurs and create companies with "explosive growth that maybe change the world". (It kinda came across like you were implying that founders with families were mostly only interested in "lifestyle businesses".)
There are so many factors that contribute to success, and they are never all equal, so any sort of up-front filtering based on some factor you think is correlated with success is going to miss out on people. And besides, I would strongly dispute any arguments that having a family is like some sort of handicap that must be overcome. In a lot of ways, I think having a family is a BETTER situation for starting a company!
My impression from reading YC's stuff is more that folks feel the value of the program comes to a large degree from the in-person interactions people have, so it makes sense to require people to be there in person.
Related, there's this scene from Moneyball, [1] that I love. They're all sitting around, discussing all this stuff that is at best tangentially related to _the actual things that matter_ for success of the team. It's very tempting and natural bias to latch on to this or that factor which you've become convinced makes a difference "all else being equal". But all else is never equal, and the question that is actually important is more the one Billy Beane keeps asking: "Yeah, but can he hit?"
[1]: https://www.youtube.com/watch?v=pWgyy_rlmag